Provided is a chain tensioner that can, with a simple structure, supply sufficient and necessary oil when restarted after a long time after the stop of oil supply, and control pressing force in accordance with operating conditions during the drive, thereby reducing vibration of the chain not only when restarted but also during the drive. In a chain tensioner 100 in which a plunger 120 inserted in a plunger bore of a tensioner body 110 is biased outward by biasing means 140 provided in an oil pressure chamber 101 to be extendable and contractible, an oil supply hole 114, an oil supply space 102, and a plunger hole 124 that together form a continuous oil supply passage is formed in such a shape that an oil amount changes in accordance with a position of the plunger 120.