Abstract: A chain tensioner (1) having a plunger (12) slidably assembled in a cylinder portion (13) together with a return spring (11), the chain tensioner (1) further has a stopper plate (22) having a through-hole (22a) formed therein, through which the plunger (12) is inserted; and a biasing member (24) disabling movement of the plunger (12), by applying a biasing force to the end of the stopper plate (22) so as to incline the stopper plate (22), the chain tensioner is configured to apply a hydraulic pressure of a hydraulic oil of an engine to the stopper plate (22), so as to reduce the inclination of the stopper plate (22) against the biasing force applied by the biasing member (24), to thereby allow movement of the plunger (12).