An object of the present invention is to provide a tensioner that enables a sufficient amount of stroke for the plunger to be secured and allows for size and weight reductions without changing the size of the tensioner body. In the tensioner according to the invention, an oil supply path that supplies oil from outside to an oil reservoir chamber formed inside the plunger includes an oil supply space formed between an inner circumferential surface of the plunger bore and an outer circumferential surface of the plunger. The oil supply space is formed by a front-end communication groove formed on an inner circumferential surface of the plunger bore and communicating with a tensioner body oil supply hole, and a rear-end communication groove formed on an outer circumferential surface of the plunger and causing the front-end communication groove to communicate with a plunger oil supply hole.