An object of the present invention is to provide a tensioner with a simple structure that effectively maintains the oil pressure inside an oil pressure chamber at a correct level, while also making the machining and assembling processes less demanding. The tensioner includes a relief mechanism, which includes an orifice member that has an oil passage designed to form an orifice structure. The orifice member has a flat circular end wall part and a circumferential wall part that extends rearwards from the peripheral edge of the end wall part. The circumferential wall part includes a bulged portion that extends rearwards and is curved radially outwards. The bulged portion is designed such that the diameter of the circumscribed circle of the orifice member is greater than the inside diameter of the plunger as viewed in plan from an axial direction before the orifice member is press-fit into the plunger.