Provided is a tensioner capable of stabilizing an oil pressure in an oil pressurizing chamber. The tensioner includes a pressure adjustment unit. The pressure adjustment unit includes a unit case attached to the housing or the plunger, a moving member disposed in the case inner space to be capable of moving closer to and farther away from the oil pressurizing chamber, and a pressure adjustment biasing unit which biases the moving member toward the oil pressurizing chamber. A gap between an inner wall of the case inner space and the moving member is configured to be held in a liquid tight state at any position between a position where the moving member is closest to the oil pressurizing chamber as a result of moving and a position where the moving member is most distant from the oil pressurizing chamber.