A chain tensioner has a plunger (14) slidably fitted in a bottomed cylinder (12) formed in a housing (11), and configured to resiliently urge the plunger (14) by a return spring (13) in the direction along which the plunger (14) is projected out from the cylinder (12), wherein the chain tensioner is configured to attach an engagement clip (19) to the opening of the cylinder (12), and to provide an engagement portion engageable with the engagement clip (19) to the end of the plunger (14) on the projection side thereof, so as to allow the engagement clip (19) and the engagement portion to engage with and disengage from each other corresponding to the position of rotation of the plunger (14).