A chain tensioner comprising: a housing in which a cylinder chamber is formed; a plunger slidably mounted in the cylinder chamber; and a return spring biasing the plunger outwardly; wherein the housing is formed with an oil supply passage communicating with a pressure chamber formed rearwardly of the plunger, and a threaded hole extending to the pressure chamber from an outer periphery of an upper portion of the housing, wherein the chain tensioner further comprises: a check valve mounted at an oil outlet side of the oil supply passage so as to prevent hydraulic oil in the pressure chamber from flowing back into the oil supply passage; a screw threadedly engaged in the threaded hole; and a spring washer mounted between a head of the screw and the outer periphery of the housing, and having circumferentially separate radial ends, and wherein the chain tensioner is configured such that air remaining in the pressure chamber can be discharged to outside of the chain tensioner through a helical gap defined between threadedly engaged portions of the screw and the threaded hole, and through between the separate radial ends of the spring washer, characterized in that at least one air discharge groove is formed in a seat surface of the head of the screw such that an annular space defined between an outer diameter surface of a neck of the screw and an inner diameter surface of the spring washer communicates with the outside through the air discharge groove. a housing in which a cylinder chamber is formed; a plunger slidably mounted in the cylinder chamber; and a return spring biasing the plunger outwardly; wherein the housing is formed with an oil supply passage communicating with a pressure chamber formed rearwardly of the plunger, and a threaded hole extending to the pressure chamber from an outer periphery of an upper portion of the housing, wherein the chain tensioner further comprises: a check valve mounted at an oil outlet side of the oil supply passage so as to prevent hydraulic oil in the pressure chamber from flowing back into the oil supply passage; a screw threadedly engaged in the threaded hole; and a spring washer mounted between a head of the screw and the outer periphery of the housing, and having circumferentially separate radial ends, and a check valve mounted at an oil outlet side of the oil supply passage so as to prevent hydraulic oil in the pressure chamber from flowing back into the oil supply passage; a screw threadedly engaged in the threaded hole; and a spring washer mounted between a head of the screw and the outer periphery of the housing, and having circumferentially separate radial ends, and wherein the chain tensioner is configured such that air remaining in the pressure chamber can be discharged to outside of the chain tensioner through a helical gap defined between threadedly engaged portions of the screw and the threaded hole, and through between the separate radial ends of the spring washer, characterized in that at least one air discharge groove is formed in a seat surface of the head of the screw such that an annular space defined between an outer diameter surface of a neck of the screw and an inner diameter surface of the spring washer communicates with the outside through the air discharge groove.