To provide a chain tensioner which ensures the compatibility of restraints on an excessively applied tensile force at a high-speed rotational movement of an engine and a chain fluttering arising at a low-speed rotational movement of an engine with oil leakage suppressed in a hydraulically pressing mechanism in a chain tensioner structured so that a tension lever, supported swingably to an engine body on a loosening side of an endless chain wound around a sprocket provided in an engine output shaft and a driven shaft, is compressed by a plunger of the hydraulically pressing mechanism.

The chain tensioner is provided with an arm mechanism 40 including a first arm unit 42b which is supported swingably to an engine body 1a between a tensioner lever 22 and the hydraulically pressing mechanism 30 and whose apical end abuts against the plunger 32, a second arm unit 43, made of a leaf spring, whose apical end abuts against the tensioner lever 22, and a stopper unit 42c provided at a given interval X on an opposite side to the tensioner lever 22 at the apical end of the second arm unit 43.