The present invention provides a chain tensioner that while being free from an increase in its size, reduces the number of processing parts and the number of assembling steps and reduces the outflow of oil to an outside. Inside a plunger, an inner sleeve including a check valve unit provided on a side of a pressure oil chamber and a pressure regulating unit provided on an outer periphery thereof is arranged. The inner sleeve has a cylinder part and a movement restricting part provided on the side of the pressure oil chamber, and the pressure regulating unit has an annular relief valve slidablyfitted onto an outer surface of the cylinder part and slidably contacting on an inner surface of the plunger and a relief spring urging the relief valve to a side of the movement restricting part.