portion of the tray 20 a tray gear 32 engaged with the first rack gear 31 and a driving motor 33 the first rack gear 31 is preferably integrally formed with the tray 20 the tray gear 32 is rotatably formed in the body 10 and moves in association with the first rack gear 31 according to rotation directions to move the tray 20 the driving motor 33 transmits power via the power transmission unit 34 to the tray gear 32 and rotates in both directions the power transmission unit 34 includes a driving pulley 35 on a rotating axis of the driving motor 33 a driven pulley 36 a belt 37 and an idle gear 38 the belt 37 connects the driving pulley 35 and the driven pulley 36 a pulley gear 36 a formed coaxially with the driven puller 36 is engaged with the idle gear 38 the power transmission unit 34 may be implemented with various examples as shown in fig3 a 3 b and 4 a the lifting unit 40 includes a pivot plate 41 pivotable up and down in the body 10 a cam slider 43 provided in the body 10 to reciprocally move in association with the pivot plate 41 a second rack gear 45 engaged with the.