An object of the present invention is to provide a chain tensioner that allows a necessary and sufficient amount of oil to be fed during restart following a long stoppage of oil feeding and that maintains a proper pressure, thus preventing possible noise and vibration. An oil storing chamber 123 is formed inside a plunger 120 and forward of a check valve 150 in a protruding direction of the plunger 120. A tensioner body 110 has an oil feeding hole 114 through which a cylindrical surface portion 113 of a plunger housing hole 111 communicates with an outside and a high-pressure discharge hole 115 through which a pressure oil chamber 101 in the plunger housing hole 111 communicates with the outside. A relief valve 160 that is opened under a high pressure is provided in the high-pressure discharge hole 115 and closer to the pressure oil chamber 101.