Provided is a chain tensioner that is capable of restricting the movement of a plunger at an engagement position, hardly causes its breakage and abrasion even if receiving a great force applied to the plunger, needs no setting for a backlash amount, and allows the engagement position of the plunger to be freely shifted forward and backward. The tensioner body has an engagement member that extends toward and retracts from engagement grooves provided on the outer periphery of the plunger, a press member that presses, in one direction, a cam member which maintains and releases the extending state of the engagement member, and a hydraulic supply path through which oil is supplied to press the cam member in a direction opposite to the pressing direction of the press member.