To provide a forward/backward driving device having a simple structure even at low cost.

This forward/backward driving device includes a fixed cylinder tube comprising partition walls which are apart from each other at the front and the rear. In the fixed cylinder tube, a working fluid such as water or air can be supplied/discharged alternatively in the front and rear directions. The driving device further includes a piston with a gasket, which can be moved forward and backward within the cylinder tube, a direction changing wheel disposed in the front and rear of the exterior of the cylinder tube, and a flexible transmission member which is connected to the front and rear sides of the piston, penetrates the partition walls as they remain internally and externally hermetically sealed, is wrapped around the direction changing wheel, and allows a forward/backward movable body to freely accompany a part between the wrapped parts. The cylinder tube serves also as a guide rail for the forward/backward movable body.