a presently preferred exemplary embodiment of the invention will be described in detail with reference to the accompanying drawings . the overall construction of the refrigerator , according to the invention , is shown in fig3 - 5 . the interior of a main body 7 of the refrigerator is divided into a freezing compartment 9 above , a refrigerating compartment 11 in the middle , and a vegetable compartment 13 below . to the front of compartments 7 , 9 , and 11 are attached adiabatic doors 15 , 17 , 19 , respectively . at the rear of freezing compartment 9 , there is formed a main evaporator compartment 21 which is separated from the freezing compartment 9 . the main evaporator compartment 21 has a main evaporator 23 in it . the interior of main evaporator compartment 21 communicates with the interior of the freezing compartment 9 through a return duct 25 formed in a heat insulation wall 27 constituting a partition between the freezing compartment 9 and the refrigerating compartment 11 , and also through a cold air supply port 29 formed in an upper portion of the main evaporator compartment 21 . a cold air circulation fan 31 is provided to the rear of the cold air supply port 29 . fan 31 pushes cold air produced by the main evaporator 23 into the freezing compartment 9 through the cold air supply port 29 , while air inside the freezing compartment 9 passes through the return duct 25 to return to the main evaporator compartment 21 . cold air produced by the main evaporator 23 is also pushed into the refrigerating compartment 11 through an air supply port of a supply duct ( not shown ) formed in a rear heat insulation wall , while air inside the refrigerating compartment 11 passes through the interior of the vegetable compartment 13 and the return duct 25 to return to the main evaporator compartment 21 . the air supply port of a supply duct ( not shown ) is provided with a damper ( not shown ) to control the temperature in the refrigerating compartment 11 . as shown in detail in fig5 in a ceiling surface portion 33 of the refrigerator main body 3 , there is provided the cold - accumulation material 35 which is enclosed in heat insulating material and has the cold - accumulation evaporator 37 embedded in it . a thermosiphon 39 provided with a electromagnetic valve 41 , as shown in fig4 connects the cold - accumulation evaporator 37 to the main evaporator 23 in a manner permitting transfer of heat as described below . the thermosiphon 39 is constituted by a closed loop pipeline which has operating fluid , such as , e . g . refrigerant , therein . the portions of the closed loop pipeline next to the main evaporator 23 and the cold - accumulation evaporator 37 are zig - zag shaped so as to enhance heat exchange . a glass - tube defrosting heater 42 is provided below the main evaporator 23 for periodic defrosting . the refrigerating cycle will be described with reference to fig2 . the discharge side of a compressor 43 is connected through a condenser 45 and a first capillary tube 47 to an inflow side of a flowpath switching type electromagnetic valve 49 . valve 49 has two outflow ports . a first of the two outflow ports connects through a second capillary tube 51 to an inflow port of the main evaporator 23 . a second of the two outflow ports connects through a third capillary tube 55 to an input of the cold accumulation evaporator 37 . an outflow port of the main evaporator 23 connects through an accumulator 53 to an intake side of the compressor 43 , whereby there is established a refrigerant flow path for ordinary cooling operation ( first mode ) to cool the main evaporator 23 and hence the interior of the compartments . cold - accumulation evaporator 37 is connected in parallel with the main evaporator 23 to the accumulator 53 , whereby there is established a refrigerant flow path for cold - accumulation mode operation ( third mode ) for cooling the cold - accumulation evaporator 37 and hence the cold - accumulation material 35 . as noted above , the thermosiphon 39 is thermally connected between the main evaporator 23 and the cold - accumulation evaporator 37 , and hence the cold - accumulation material . it is arranged in such a way that a cold - accumulation material cooling operation can be effected , in which the main evaporator 23 and hence the interior of compartments are cooled by exchange of heat between the main evaporator 23 and the cold - accumulation material 35 when the electromagnetic valve 41 is opened . fig1 shows significant portions of the control circuit of the refrigerator according to the present invention . a single chip miorocomputer 57 executes programs stored in a rom ( not shown ), and controls energization and deenergization of relays 59 , 61 , 63 , 65 in accordance with output timing signals from a clock circuit 67 , a signal from a room temperature detection circuit 69 , etc . providing &# 34 ; high &# 34 ; logic signals to the bases of transistors 71 to 77 , respectively connected to the relays 59 to 65 , results in energization of relays 59 to 65 , respectively . when first relay 59 is energized , a contact ( not shown ) is closed and as a result the compressor 43 is actuated by a commercial power supply or an invertor device outputting , e . g ., 120 hz ac power . when the second relay 61 is energized , a contact ( not shown ) is closed and as a result power is supplied to the electromagnetic valve 41 , causing it to assume a position permitting movement of operating fluid in thermosiphon 39 and heat exchange between the cold - accumulation material 35 and the main evaporator 23 . when the third relay 63 is energized , a contact ( not shown ) is closed and as a result , power is supplied to the valve 49 , whereby a switch from a first flowpath for ordinary cooling operation ( first mode ) to a second flowpath for the cold - accumulation operation . when the fourth relay 65 is energized , a contact ( not shown ) is closed , and as a result the cold air circulation fan 31 is actuated , whereby cold air is circulated in the compartments . a freezer sensor 79 , as is well known , comprises a thermistor having negative temperature coefficient . one end of the freezer sensor 79 is connected to a d . c . power supply vcc and the other end is connected to ground through a resistor 81 . a connection point between the freezer sensor 79 and the resistor 81 is connected to a temperature detecting circuit 83 . when the compartment interior temperature detection by the freezer sensor 79 rises above a prescribed level , such as , e . g ., - 19 ° c ., the temperature detection circuit 83 outputs a &# 34 ; high &# 34 ; logic signal to one of the input ports of the microcomputer 57 , and ordinary cooling operation or cold - accumulation material cooling operation is carried out . a room temperature detection circuit 85 includes a room temperature sensor 87 and an a / d converter 89 . the room temperature sensor 87 is preferably a thermistor having negative temperature coefficient which detects the ambient room temperature . a / d converter 89 digitizes an output analog voltage from the room temperature sensor 87 , and provides it to one of the input ports of the microcomputer 57 . ordinary cooling is carried out by causing compressor 43 to supply refrigerant to the main evaporator 23 . power to the second relay 61 and the third relay 63 is cut - off by the microcomputer 57 which causes a &# 34 ; low &# 34 ; signal to be provided to the bases of the second transistor 73 and third transistor 75 , whereby the electromagnetic valve 41 is closed , and the electromagnetic valve 49 is deactivated . as a result , thermosiphon 39 ceases to operate . the refrigerant flowpath in the refrigerating cycle is switched to the ordinary cooling operation flowpath . when the temperature in the freezing compartment 9 rises , and the temperature detecting circuit 83 outputs a &# 34 ; high &# 34 ; signal to one of the input ports of the microcomputer 57 , the first relay 59 and the forth relay 65 are energized by the microcomputer 57 causing &# 34 ; high &# 34 ; signals to be provided to the bases of the first transistor 71 and the forth transistor 77 . as the first relay 59 and the fourth relay 65 are energized , the compressor 43 and the cold air circulation fan 31 are actuated by a commercial power supply . as a result , refrigerant is supplied to the main evaporator 23 and cold air produced thereby is circulated by the cold air circulation fan 31 to cool the refrigerator compartments . when the temperature in the freezing compartment 9 falls to the prescribed value , the &# 34 ; high &# 34 ; signal from the temperature detecting circuit 83 is cut off , and the first relay 59 and the fourth relay 65 are deenergized by the microcomputer 57 . the &# 34 ; high &# 34 ; signals are no longer applied to the bases of the first transistor 71 and the fourth transistor 77 . as a result , ordinary cooling operation is stopped . in this manner , the interior temperature of compartments are individually kept below a set temperature by the ordinary cooling operation . in second mode operation , the refrigerator compartments are cooled by means of the cold - accumulation material . heat is exchanged between the cold - accumulation material 35 and the main evaporator 23 . power to the first relay 59 is cut off by the microcomputer 57 by outputting a &# 34 ; low &# 34 ; signal to the base of the first transistor 71 and power to the third relay 63 is supplied by the microcomputer 57 causing a &# 34 ; high &# 34 ; signal to be provided to the base of the third transistor 75 , whereby the compressor 43 is maintained deactuated and the valve 49 is activated . as a result , the refrigerant flowpath in the refrigerating cycle is switched from the flowpath for the ordinary cooling operation to the flowpath for cold - accumulation operation . when the temperature in the freezing compartment 9 rises , and the temperature detecting circuit 83 outputs a &# 34 ; high &# 34 ; signal to one of the input ports of the microcomputer 57 , power is supplied to the second relay 61 and the fourth relay 65 when microcomputer 57 outputting h - level signals to the bases of the second transistor 73 and the fourth transistor 77 . when the second relay 61 and the fourth relay 65 are energized , the electromagnetic valve 41 is opened and the cold air circulation fan 31 is actuated by the commercial power supply . as a result , heat exchange between the main evaporator 23 and the cold - accumulation material 35 is permitted . an operating fluid , preferably a refrigerant but not necessarily so , enclosed in the pipeline of the thermosiphon 39 absorbs heat from the main evaporator 23 , where the operating fluid is evaporated from a liquid state to a gas state . the gas passes along the pipeline of the thermosiphon 39 , and rises to the cold - accumulation material 35 section , wherein the operating fluid gas is cooled and condenses to a liquid , and then travels along the pipeline to return to the main evaporator 23 . there , the operating fluid again absorbs heat of the freezer interior . cold air produced by the main evaporator 23 is circulated by the cold air circulation fan 31 , thereby cooling the refrigerator compartments . when the temperature in the freezing compartment 9 falls to the prescribed value , such as , e . g ., - 22 ° c ., the &# 34 ; high &# 34 ; signal from the temperature detecting circuit 83 is cut off , and the second relay 61 and the fourth relay 65 are deenergized by the microcomputer 57 by its causing the &# 34 ; high &# 34 ; signals to be removed from the bases of the second transistor 73 and the fourth transistor 77 . as a result , the electromagnetic value 41 is closed , the cold air circulation fan is deactuated , and cooling by means of the cold - accumulation material ceases . in this manner , the interior of compartments are individually kept below the set temperature by the cold - accumulation material cooling operation . as made clear below , the cold - accumulation material cooling operation can be performed only during a set time band in the daytime . in third mode operation , the cold - accumulation material is cooled by supplying refrigerant to the cold - accumulation evaporator 37 during a predetermined time interval ( usually at night ) when power demand is low . power to the second relay 61 is cut off by microcomputer 57 causing a &# 34 ; low &# 34 ; signal to be applied to the base of the second transistor 73 . power to the third relay 63 is supplied by the microcomputer 57 causing a &# 34 ; high &# 34 ; signal to be applied to the base of the third transistor 75 . when the second relay 61 is denergized , and the third relay 63 is energized and valve 49 is activated . as a result , the refrigerant flowpath is switched from the flowpath for the ordinary cooling operation to the flowpath for the cold - accumulation operation . while these conditions exist , microcomputer 57 causes a &# 34 ; high &# 34 ; signal to be applied to the base of the first transistor 71 which , in turn , causes the first relay 59 to be energized . this couples compressor 43 to an invertor unit ( not shown ) outputting 720 hz ac power which causes the compressor to be operated at a higher capacity than it would otherwise operate with when connected to an ordinary commercial power supply . refrigerant is supplied to the cold - accumulation evaporator 37 , whereby the cold - accumulation evaporator 37 and hence cold - accumulation material 35 are cooled . during this cold - accumulation operation , if the interior temperature of compartments rises above the prescribed valve , the cold - accumulation operation is temporarily halted and the above - described ordinary cooling operation is effected to cool the compartment interiors . the cooling capacity of the cold - accumulation material 35 is such that it is sufficient even if the cold - accumulation material cooling operation is carried out frequently in high - temperature situations as in summer , etc . consequently , the cooling capacity of the cold - accumulation material 35 tends to be excessive at times of low - temperature when the frequency of execution of the cold - accumulation material cooling operation is less . in this embodiment , therefore , the arrangement is as follows . as is shown in fig6 the microcomputer 57 effects control such that in the period from 8 : 00 a . m . to 1 : 00 p . m . the compartment interior is cooled by the above - described ordinary cooling operation when the compartment interior temperature rises above the prescribed valve . further , control is such that in the period from 1 : 00 p . m . to 4 : 00 p . m ., the compartment interior is cooled by the above - described cold - accumulation material cooling operation when the compartment interior temperature rises above the prescribed valve . also , from 1 : 00 p . m . to 4 : 00 p . m . the average room temperature is calculated . if the average room temperature from 1 : 00 p . m . to 4 : 00 p . m . is , e . g ., 15 ° c . or more , execution of the ordinary cooling operation instead of the cold - accumulation material cooling operation is made possible , as indicated in fig6 -( a ). in this case , if the average room temperature from 1 : 00 p . m . to 4 : 00 p . m . is 15 ° c . or more , the time band in which the cold - accumulation material cooling operation is performable is the time band from 1 : 00 p . m . to 4 : 00 p . m . subsequently , during the period from 4 : 00 p . m . to 10 : 00 p . m ., ordinary cooling is carried out . during the period from 10 : 00 p . m . to 8 : 00 a . m . on next day cold - accumulation operation is executed . however , if the average room temperature during 1 : 00 p . m . to 4 : 00 p . m . is , e . g ., less than 15 ° c ., the microcomputer 57 extends the time band in which the cold - accumulation material cooling operation is performable , making an adjustment so that it lasts up to , for example , 6 : 00 p . m ., as indicated in section b of fig6 . in this case , if the average room temperature from 1 : 00 p . m . to 4 : 00 p . m . is less than 15 ° c ., the time band in which the cold - accumulation material cooling operation is performable is the time band from 1 : 00 p . m . to 6 : 00 p . m . of a day . subsequently , during the period from 6 : 00 p . m . to 10 : 00 p . m ., ordinary cooling takes place . during the period from 10 : 00 p . m . to 8 : 00 a . m . on the next day , the cold - accumulation material is cooled . if , for example , the average room temperature from 1 : 00 p . m . to 4 : 00 p . m . is lower than 15 ° c ., the time band in which cooling by means of the cold - accumulation material is extended by 2 hours . the cold - accumulation material 35 which , at 4 : 00 p . m ., still has remaining cooling capacity because of the low room temperature can still exchange heat with the main evaporator 23 through the thermosiphon 39 . thus , the cooling capacity of the cold - accumulation material is put to effect use . the cooling of the cold accumulation material is delayed so that more of the cooling capacity of the cold - accumulation material 35 can be used . the cold accumulation material is not so much needlessly cooled and power is not wasted . the present invention has been described with respect to a specific embodiment . however , other embodiments based on the principles of the present invention should be obvious to those of ordinary skill in the art . for example , when the time band for effecting the cold - accumulation material cooling operation is extended , in order to still further ensure refrigerator compartment cooling in the extended time band , a cold - accumulation material temperature sensor may be provided near the cold - accumulation material to sense the cold - accumulation material cooling capacity . changeover to permit refrigerator compartment by the ordinary cooling operation is made if the detected cooling capacity is insufficient . such embodiments are intended to be covered by the claims .