hereinafter , preferred embodiments of the present invention are described referring to the drawings . fig1 is a circuit diagram illustrating a constitution of a semiconductor storage device according to a preferred embodiment 1 of the present invention . bit lines bl and / bl are connected to sources of a pair of access transistors in a memory cell 1 of sram ( static random access memory ) activated by access from a word line wl . a precharge circuit 2 , an equalizing circuit 3 and a reading circuit 4 are connected to the bit lines bl and / bl . the equalizing circuit 3 comprises an equalizing transistor qp 3 . a pmos transistor constitutes the equalizing transistor qp 3 . a source and a drain of the equalizing transistor qp 3 are connected to the bit lines bl and / bl , and an equalizing control signal eq is applied to a gate thereof . the precharge circuit 2 comprises switching transistors qp 1 and qp 2 , which are pmos transistors serving as precharge switching elements , and a power supply connecting circuit 5 . a ground connecting circuit 6 , which is a step - down circuit , is connected to the bit lines bl and / bl with the precharge circuit 2 interposed therebetween . a source of the precharge transistor qp 1 is connected to the bit line bl , and a source of the precharge transistor qp 2 is connected to the bit line / bl . a gate of the precharge transistor qp 1 and a gate of the precharge transistor qp 2 are connected to each other , and further connected to the gate of the equalizing transistor qp 3 . a drain of the precharge transistor qp 1 and a drain of the precharge transistor qp 2 are connected to each other , thereby serving as a control node nc . the control node nc is connected to a high - potential - side power supply ( vdd ) via the power supply connecting circuit 5 , and further connected to a low - potential - side power supply ( gnd ) via the ground connecting circuit 6 . the power supply connecting circuit 5 is turned on and off by a precharge control signal pc to thereby connect / disconnect the control node nc with respect to the high - potential - side power supply . the ground connecting circuit 6 is turned on and off by a step - down control signal dc to thereby connect / disconnect the control node nc with respect to the low - potential - side power supply . the on - off control by the power supply connecting circuit 5 and the on - off control by the ground connecting circuit 6 are related to each other in a trade - off manner . the ground connecting circuit 6 constitutes a main constituent of a step - down function . the main constituent of the step - down function is not directly connected to the bit lines bl and / bl , but is connected to the bit lines bl and / bl with the switching transistors qp 1 and qp 2 interposed therebetween . the present invention is characterized in that the main constituent of the step - down function is thus provided in the bit lines bl and / bl with the switching transistors qp 1 and qp 2 interposed therebetween . because of the constitution thus described , load capacities of the bit lines bl and / bl can be prevented from increasing . fig2 is a circuit diagram specifically illustrating the power supply connecting circuit 5 and the ground connecting circuit 6 shown in fig1 . a pmos precharge transistor qp 0 constitutes the power supply connecting circuit 5 , and an nmos step - down transistor qn 0 constitutes the ground connecting circuit 6 . a source of the precharge transistor qp 0 in the power supply connecting circuit 5 is connected to the high - potential - side power supply , a drain thereof is connected to the control node nc , and the precharge control signal pc is applied to a gate thereof . a source of the step - down transistor qn 0 in the ground connecting circuit 6 is connected to the low - potential - side power supply , a drain thereof is connected to the control node nc , and the step - down control signal dc is applied to a gate thereof . an operation of the semiconductor storage device thus constituted according to the present preferred embodiment is described referring to a timing chart shown in fig3 . at a timing t 0 , the low - active precharge control signal pc is in assert state , the step - down control signal dc is in negate state , and the low - active equalizing control signal eq is in assert state . because the precharge control signal pc is at “ l ” level , the precharge transistor qp 0 is in the on state , and a potential of the control node nc is the power supply voltage vdd . further , the equalizing control signal eq is at “ l ” level ; therefore , the switching transistors qp 1 and qp 2 and the equalizing transistor qp 3 is in the on state . accordingly , the power supply voltage vdd of the control node nc is applied to the bit lines bl and / bl , and the bit lines bl and / bl are thereby precharged . prior to the activation of the word line wl ( t 3 ), at a timing t 1 , the precharge control signal pc is negated to turn to “ h ” level , and the precharge transistor qp 0 is thereby turned off . then , the control node nc is disconnected from the power supply voltage vdd , which leaves the bit lines bl and / bl in a floating state . at the time , the switching transistors qp 1 and qp 2 remain in the on state . at a timing t 2 , the step - down control signal dc is asserted to turn to “ h ” level . then , the step - down transistor qn 0 in the off state so far is turned on , and a potential of the control node nc is stepped down to the ground level . because the switching transistors qp 1 and qp 2 are in the on state at the time , the voltages of the bit lines bl and / bl are stepped down in response to the potential drop in the control node nc . the potentials of the bit lines bl and / bl are stepped down along with a certain time constant and to a predetermined voltage level . a possible example of the predetermined voltage level is vdd − vth . vth is a threshold voltage of the mos transistors . at the time , a step - down speed in the bit line is lower as the voltage is closer to the predetermined voltage . therefore , variability in a time length from the time when the step - down transistor qn 0 is turned on to the time when the switching transistors qp 1 and qp 2 are turned on and variability in the step - down level resulting from the characteristic variability of the step - down transistor qn 0 can be controlled . at a timing t 3 , the equalizing control signal eq is negated to turn to “ h ” level . at the time , the switching transistors qp 1 and qp 2 are turned off , and the step - down transistor qn 0 is thereby completely disconnected from the bit lines . immediately after that , the word line wl is activated to turn to “ h ” level . when the equalizing control signal eq turns to at “ h ” level , the switching transistors qp 1 and qp 2 are turned off and thereby disconnected from the ground , which stops the step - down operation for the bit lines bl and / bl . further , the equalizing transistor qp 3 is turned off , which stops the equalizing operation for the bit lines bl and / bl . since the word line wl is at “ h ” level , data is read from the memory cell 1 . the reading operation at the time is similar to that of the conventional technology . according to the present preferred embodiment , the step - down transistor qn 0 , which is the main constituent of the step - down function , is not directly connected to the bit lines bl and / bl , and the switching transistors qp 1 and qp 2 are interposed therebetween . accordingly , the load capacities of the bit lines bl and / bl can be prevented from increasing . further , during the reading operation , the time constant used when the bit lines bl and / bl shift from the power supply voltage vdd to the ground level is lessened so that the data can be read at a high speed . provided that an amount of time necessary for the data read in the conventional technology is tu and an amount of time necessary for the data read according to the present invention is ta , ta & lt ; tu . the pmos transistors are used as the switching transistors qp 1 and qp 2 . accordingly , during the step - down operation , when the voltages of the bit lines bl and / bl are stepped down , source − drain voltages in the switching transistors qp 1 and qp 2 are reduced , and the step - down capacities of the pmos transistors qp 1 and qp 2 are lessened . as a result , variability of the step - down levels in the bit lines can be effectively alleviated in the case where a timing of terminating the step - down control varies . fig4 a is a circuit diagram illustrating a constitution of a semiconductor storage device according to a preferred embodiment 2 of the present invention . fig5 is a circuit diagram illustrating an equivalent circuit shown in fig4 a . the gate of the precharge transistor qp 0 and the gate of the step - down transistor qn 0 are connected to each other , and these transistors qp 0 and qn 0 constitute an inverter inv . the precharge transistor qp 0 and the step - down transistor qn 0 are controlled by a precharge / step - down control signal pdc which is a control signal common to them . an operation of the semiconductor storage device thus constituted according to the present preferred embodiment is described referring to a timing chart shown in fig4 b . at a timing t 10 , the precharge / step - down control signal pdc is at “ l ” level , and the low - active equalizing control signal eq is in assert state . because the precharge / step - down control signal pdc is at “ l ” level , the precharge transistor qp 0 is in the on state , while the step - down transistor qn 0 is in the off state , and the potential of the control node nc is accordingly the power supply voltage vdd . because the equalizing control signal eq is at “ l ” level , the switching transistors qp 1 and qp 2 and the equalizing transistor qp 3 is in the on state . accordingly , the power supply voltage vdd of the control node nc is applied to the bit lines bl and / bl , and the bit lines bl and / bl are precharged . prior to the activation of the word line wl ( t 12 ), at a timing t 11 , the precharage / step - down control signal pdc turns to “ h ” level , and as soon as the precharge transistor qp 0 is turned off , the step - down transistor qn 0 is turned on . accordingly , the control node nc is disconnected from the power supply voltage vdd and connected to the ground at the same time . at the time , the switching transistors qp 1 and qp 2 are in the on state ; therefore , the voltages of the bit lines bl and / bl are stepped down in response to the potential drop of the control node nc . the potentials of the bit lines bl and / bl are stepped down along with a certain time constant and to a predetermined voltage level ( vdd − vth ). at a timing t 12 , the equalizing control signal eq is negated to turn to “ h ” level , and the word line wl is activated to turn to “ h ” level . when the equalizing control signal eq is at “ h ” level , the switching transistors qp 1 and qp 2 are turned off , and thereby disconnected from the ground , which stops the step - down operation for the bit lines bl and / bl . further , the equalizing operation for the bit lines bl and / bl also stops since the equalizing transistor qp 3 is turned off . since the word line wl is at “ h ” level , data is read from the memory cell 1 . at a timing t 13 , the word line wl is at “ l ” level , and the data reading operation is terminated . at a timing t 14 , the precharge / step - down control signal pdc turns to “ l ” level , and the control node nc is precharged with the power supply voltage . at the same time , the equalizing control signal eq is asserted , and the switching transistors qp 1 and qp 2 and the equalizing transistor qp 3 are turned on . accordingly , the bit lines bl and / bl are precharged with the power supply voltage . according to the present preferred embodiment , the precharge / step - down control signal pdc is shared for the control signal for the power supply connecting circuit 5 ( precharge transistor qp 0 ) and the control signal for the ground connecting circuit 6 ( step - down transistor qn 0 ), which improves an area reduction . further , the on - off control of the power supply connecting circuit 5 and the ground connecting circuit 6 is perfomed at the same time . therefore , variability in the step - down level and through current can be controlled even if there is a variation in the timing between the turn - off of the power supply connecting circuit 5 and the turn - on of the ground connecting circuit 6 or between the turn - on of the power supply connecting circuit 5 and the turn - off of the ground connecting circuit 6 . in the preferred embodiment 1 , the control signals for the precharge circuit 2 are the precharge control signal pc and the step - down control signal dc . in the present preferred embodiment , however , only the prechareg / step - down control signal pdc is used . as a result , in the precharge circuit 2 , the influence of setup on input signals is lessened . fig6 a is a circuit diagram illustrating a constitution of a semiconductor storage device according to a preferred embodiment 3 of the present invention . the inverter inv is connected equally to the control nodes nc in the precharge circuits 2 provided with the step - down function which are provided in a group of bit lines bl and / bl in a plurality of memory cells 1 parallel - arranged in a column direction . more specifically describing the constitution , the power supply connecting circuit 5 ( precharge transistor qp 0 ), ground connecting circuit 6 ( step - down transistor qn 0 ) and precharge / step - down control signal pdc are shared among the group of bit lines bl and / bl . an operation according to the present preferred embodiment is similar to that of the preferred embodiment 2 . according to the present preferred embodiment , wherein the constituent elements are shared , a layout size can be largely reduced . while there has been described what is at present considered to be preferred embodiments of this invention , it will be understood that various modifications may be made therein , and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of this invention .