A chain tensioner includes a cylinder inserted in a tensioner mounting hole of an engine cover and having a flange fixed to the outer surface of the engine cover. A plunger is axially slidably mounted in the cylinder. The cylinder is formed with an oil supply passage through which hydraulic oil is introduced into a pressure chamber defined by the cylinder and the plunger. The oil supply passage includes a first hole portion extending inwardly from a mating surface of the flange facing the engine cover, and a second hole portion extending from the outer periphery of the flange to the pressure chamber and intersecting the first hole portion. An externally threaded portion of an externally threaded member is in threaded engagement with an internal thread formed on the second hole portion, thereby closing its outer open end. The externally threaded member is fixed in position with an adhesive applied to its externally threaded portion.