A chain tensioner is proposed which includes: a pressure chamber (18) of which the volume changes with axial movement of a plunger (10) in a tubular cylinder (9); a check valve disposed between the interior of the plunger (10) and the pressure chamber (18); a communication path (30) through which a leakage gap (19) between the plunger (10) and the cylinder (9) communicates with the interior of the plunger (10); and a reverse flow check valve (40) disposed in an oil supply path (31) that connects the inside of the cylinder (9) to the outside thereof, and restricts the flow of oil from inside to outside the cylinder (9).