Provided, is a simple-structured tensioner that can. favorably expel the air that has entered an oil pressure chamber, while enabling a reduction in size in the axial direction, and in processing and assembling workload. The tensioner 10 includes a relief mechanism made up of a relief hole 22 and a disc-like orifice member 60. A first main surface 61 of the orifice member 60 is provided with an outer circumferential annular groove 61a formed along an outer peripheral edge and a communication groove 61b that extends from a central portion of the first main surface 61 to the outer circumferential annular groove 61a, The orifice member 60 has a plurality of notches 63 arranged along a circumferential direction in the outer peripheral edge of the orifice member 60.