To provide a tensioner capable of keeping oil pressure in an oil pressure chamber stable with a simple structure. The tensioner includes a pressure adjusting mechanism that adjusts oil pressure in the oil pressure chamber. The pressure adjusting mechanism includes a cylinder part, a piston unit set inside the cylinder part so as to be able to approach and separate from the oil pressure chamber, and a piston biasing unit. Between the cylinder part and the piston unit, a liquid tight condition is kept at any place between a position where the piston unit has moved closest to the oil pressure chamber and a position where the piston unit has moved closest to the adjustment space.