a preferred embodiment of the present invention will now be described in detail with reference to the accompanying drawings . referring to fig1 a refrigerator for keeping foods in cold storage is shown which comprises an insulated main body 10 and a storage chamber 11 . the storage chamber 11 is open at its front portion . slidably mounted inside the storage chamber 11 are one or more drawer - type storage containers 12 which are typically filled with food . this slidably mounting is provided by a pair of sliding devices ; one sliding device , is denoted 20 and the other corresponding sliding device is hidden from view on the opposite side of the storage container 12 . the two sliding devices are collectively referred to as sliding devices 20 hereinbelow . the sliding devices 20 enable the storage containers 12 to slide backwardly and forwardly and , as explained below , are mounted on the storage chamber 11 and the corresponding storage container 12 . as shown in fig2 each sliding device 20 comprises a fixed rail 30 which is fixedly mounted to an inner side surface of the storage chamber 11 of the refrigerator body 10 . the fixed rail 30 extends axially outwardly from this surface in the direction of a sliding motion of the storage container 12 . a movable rail 40 is mounted on an outer side surface of the storage container 12 so as to slide along the fixed rail 30 . a middle or intermediate rail 50 is slidably mounted between the fixed rail 30 and the movable rail 40 and provides coupling shown between the fixed rail 30 and the movable rail 40 . because each of the pair of sliding devices 20 mounted on opposite sides of the storage container 12 is of the same construction , only one sliding device will be described hereinafter in detail . referring to fig3 and 4 , the fixed rail 30 , which is made of an injection molded plastic , includes a rail receiving portion 38 which has a flattened c - shaped traverse cross section . the rail receiving portion 38 includes a guiding space 33 and a roller coupling portion 32 . the roller coupling portion 32 extends downward from the rail receiving portion 38 to form a receiving space 32 a into which a first plurality of rollers 31 are received . an upper rail portion 51 of the middle rail 50 , which is described hereinbelow , is slidably received in the guiding space 33 of the rail receiving portion 38 . the rollers 31 are disposed in series in a roller supporting member 34 . the roller supporting member 34 is of a predetermined length and is open at its upper and lower portions . the roller supporting member 34 receiving the rollers 31 therein is received in the receiving space 32 a of the roller coupling portion 32 . the movable rail 40 is of the same construction as the fixed rail 30 , and is mounted on a side surface of the storage container 12 so as to slide with the storage container 12 . the movable rail 40 includes a roller coupling portion 42 at an upper portion thereof , and a rail receiving portion 48 at a lower portion thereof . accordingly , the roller coupling portion 42 and the rail receiving portion 48 of the movable rail 40 are disposed in a reversed orientation with respect to the corresponding portions 32 and 38 of the fixed rail 30 . a second plurality of rollers 41 is disposed in series in a roller supporting member 44 . the coupling structure thereof corresponds to the coupling structure of the rollers 31 . the roller supporting member 44 , with the second rollers 41 received therein , is inserted into , i . e ., received in , a receiving space 42 a of the roller coupling portion 42 of the movable rail 40 . the first and second rollers 31 and 41 , which are respectively mounted on the fixed rail 30 and the movable rail 40 , as well as the roller supporting members 34 and 44 , are made of an injection molded plastic . this prevents sliding device 20 from becoming rusty when used in the humid storage chamber 11 of a refrigerator or the like . accordingly , the storage container 12 slides smoothly , and , moreover , the sliding device 20 can be easily manufactured . the middle rail 50 has an s - shaped transverse cross section and is coupled to the fixed rail 30 and movable rail 40 . the middle rail 50 includes an upper rail portion 51 formed with an upper channel 52 for receiving the roller coupling portion 42 of the movable rail 40 , and for guiding the sliding motion of the movable rail 40 . the middle rail 50 also includes a lower rail portion 53 formed with a lower channel 54 for receiving the roller coupling portion 32 of the fixed rail 30 therein , and for enabling the middle rail 50 to slide by virtue of the rolling motion of the first rollers 31 of the fixed rail 30 . additionally , a partition rail portion 55 connects the upper rail portion 51 with the lower rail portion 53 and forms a partition between the upper channel 52 and the lower channel 54 . the partition rail portion 55 is formed between the upper and lower rail portions 51 and 53 . in other words , the upper channel 52 and the lower channel 54 are respectively disposed above and below the partition rail portion 55 . the middle rail 50 is also made of an injection molded plastic . after the fixed rail 30 and the movable rail 40 are coupled to the middle rail 50 , the upper rail portion 51 of the middle rail 50 is slidably inserted into the guiding space 33 of the rail receiving portion 38 of the fixed rail 30 , as shown in fig4 . further , the lower rail portion 53 of the middle rail 50 is slidably inserted into the guiding space 43 of the rail receiving portion 48 of the movable rail 40 . in addition , the roller coupling portion 32 of the fixed rail 30 is disposed in the lower channel 54 of the middle rail 50 , and the roller coupling portion 42 of the movable rail 40 is slidably disposed in the upper channel 52 . slots 35 and 45 , through which the rollers 31 and 41 pass , are formed lengthwise at the corresponding roller coupling portions 32 and 42 of the fixed rail 30 and the movable rail 40 , respectively so that the first and second rollers 31 and 41 can roll when in contact with the partition rail portion 55 of the middle rail 50 when the rails 30 , 40 and 50 are assembled as aforementioned . stated differently , the first and second rollers 31 and 41 protrude toward the respective guiding spaces 33 and 43 through the slots 35 and 45 formed at the fixed and movable rails 30 and 40 , when the middle rail 50 is coupled to the fixed and movable rails 30 and 40 , and thus the first and second rollers 31 and 41 are disposed in contact with the partition rail portion 55 of the middle rail 50 . guiding grooves 55 a and 55 b for guiding the rolling motion of the first and second rollers 31 and 41 are formed lengthwise at the upper and lower surfaces of the partition rail portion 55 , respectively . each sliding device 20 further includes a first movement restricting means between the fixed rail 30 and the middle rail 50 and a second movement restricting means between the middle rail 50 and the movable rail 40 for restricting the movement of the middle rail 50 and for restricting the movement of the movable rail 40 , respectively . the first means , i . e ., the means for restricting the movement of the middle rail 50 , includes an upper hooking member or hook 56 which protrudes upward from upper rear end portion of the upper rail portion 51 of the middle rail 50 , and protrudes from an upper opening 36 . the upper opening 36 extends lengthwise for a predetermined distance in an upper surface of the fixed rail 30 . accordingly , as the middle rail 50 slides backwardly and forwardly , the upper hooking member 56 engages the front end of the upper opening 36 , thereby stopping movement of the middle rail 50 . the second means , i . e ., the means for restricting the range of motion or moving distance of the movable rail 40 includes a lower hooking member 57 which protrudes downwardly from a lower front end portion of the lower rail portion 53 of the middle rail 50 , and protrudes from a lower opening 47 . the lower opening 47 extends lengthwise for a predetermined length in a lower surface of the movable rail 40 . accordingly , as the moving rail 40 slides backwardly and forwardly , the lower hooking member 57 engages the rear end of the lower opening 47 , thereby stopping movement of the movable rail 40 . in operation , when the rails 30 , 40 and 50 are assembled , i . e ., coupled together as shown in fig4 the storage container 12 is slidably mounted in the storage chamber 11 and the rails 30 , 40 and 50 are located substantially one over another as shown in fig5 . the first rollers 31 are received in the lower channel 54 of the middle rail 50 and are supported by the roller coupling portion 32 ( fig4 ) of the fixed rail 30 . the first rollers 31 are located at a frontal open portion of the storage chamber 11 . the second rollers 41 are received in the upper channel 52 of the middle rail 50 and are supported by the roller coupling portion 42 ( fig4 ) of the moving rail 40 . as illustrated , the second rollers 41 are located at a rear closed portion of the storage chamber 11 . when the storage container 12 is pulled forwardly to open the same , the movable rail 40 which is mounted on the side surface of the storage container 12 , is moved forward . consequently , the second rollers 41 of the movable rail 40 roll along the guiding groove 55 a ( fig4 ) formed at the lower surface of the upper channel 52 of the middle rail 50 . accordingly , movable rail 40 is moved forward with the storage container 12 . as shown in fig6 when the storage container 12 is in a half opened position , the lower hooking member 57 of the middle rail 50 is located adjacent to a rear portion of the lower opening 47 of the movable rail 40 . the second rollers 41 move toward a front portion of the upper channel 52 which is disposed above the first rollers 31 . when the storage container 12 is pulled forward , the lower hooking member 57 of the middle rail 50 engages the rear end of the lower opening 47 of the movable rail 40 thereby stopping further movement of the movable rail 40 . if , at this stage , the storage container 12 is pulled forwardly , the lower hooking member 57 , with which the rear end of the lower opening 47 is engaged , is also pulled forwardly . accordingly , the middle rail 50 moves forward with the storage container 12 while the first rollers 31 roll without moving . as shown in fig7 when the middle rail 50 moves forwardly a predetermined distance corresponding to that at which the storage container 12 is fully opened , the upper hooking member 56 engages the front end of the upper opening 36 of the fixed rail 30 , thereby stopping the further movement of the middle rail 50 . if a user pushes the storage container 12 into the storage chamber 11 after putting food into or withdrawing food from the storage container 12 , the storage container 12 slides into the storage chamber 11 in an operation which is the reverse of that associated with pulling out the storage container 12 . during the sliding motion of the storage container 12 described above , the weight of the storage container 12 is transferred to the partition rail portion 55 of the middle rail 50 through the second rollers 41 of the movable rail 40 , and thereafter to the first rollers 31 of the fixed rail 30 . finally , the weight of the container 12 is transferred to a side wall of the storage chamber 11 on which the fixed rail 30 is mounted . it will be understood from the foregoing that during the sliding motion of the sliding device 20 , the first and second rollers 31 and 41 contact and roll on the partition rail portion 55 of the middle rail 50 . further , the weight of the storage container 12 is transferred proportionally to the first and second rollers 31 and 41 . this operation provides smoothing out of the sliding motion of the middle rail 50 and the movable rail 40 ( see fig4 ). alternatively , the movable rail 40 and the middle rail 50 may be moved in reverse order or simultaneously . however , the fundamental operation of the respective rails 40 and 50 remains the same . because the fixed rail 30 , the movable rail 40 , the middle rail 50 and the plural rollers 31 and 41 are made of an injection molded plastic , the sliding device 20 does not rust nor is device 20 prone to other problems associated with metal parts in a humid environment . therefore , the device 20 can be used in the humid storage chamber 11 of a refrigerator or the like without impairing the operation of the sliding device 20 . further , because the first and second rollers 31 and 41 roll with minimum contact in the guiding grooves 55 a and 55 b formed in the partition rail portion 55 , the rollers 31 and 41 can roll over granular ice which may be produced on a surface of the middle rail 50 , thereby ensuring smooth movement of the storage container 12 . although the invention has been described above in relation to preferred embodiments thereof , it will be understood by those skilled in the art that variations and modifications can be effected in these preferred embodiments without departing from the scope and spirit of the invention .