A tensioner includes a base, a piston, a rack, a stopping device, a first and a second flexible device. The surface of the base includes a first, a second, a third, and a fourth recess portion vertically connecting the third with the second recess portion. The rack disposed in the second recess portion has one ending outside the base to extend and connect to one ending of the piston. The stopping device axially disposed in the fourth recess portion has a work surface and a first connecting surface disposed on sides of two endings thereof. Two sticking portions are disposed on opposite edges of the first connecting surface. A turning mechanism is disposed on the ending of stopping device, which has the sticking portions. The second flexible device disposed in the third recess portion contacts the ending of stopping device, where the work surface is disposed.