To provide a chain tensioner that has a simple structure, is universally usable and capable of supplying sufficient and necessary oil when restarted after a long time after the stop of oil supply, and enables reduction of its length and size. Urging unit 140 provided in an oil pressure chamber 101 to be extendable and contractible urges a plunger 120 outward from a plunger bore 111 in a tensioner body 110 of the chain tensioner 100. A connection groove 122 is formed in the outer circumference of the plunger 120 to form a supply chamber 102. An oil reservoir chamber 123 is formed inside the plunger 120. A leak groove 125 is formed in the outer circumference of the plunger 120 on the side of the oil pressure chamber 101 such as to extend into the supply chamber 102.