Tensioner

A tensioner comprising:

• a plunger having a plunger hole that is open on a rear side;
• a housing having a plunger bore that is open on a front side and accommodates the plunger;
• main biasing means that is accommodated inside an oil pressure chamber formed between the plunger and the plunger bore such as to freely expand and contract, and that biases the plunger toward the front side;
• and
• a relief valve mechanism that releases oil inside the oil pressure chamber out of the plunger when oil pressure in the oil pressure chamber rises,
• the relief valve mechanism including a relief valve unit inserted into the plunger hole on a front side thereof, and an external relief hole that causes a relief-side space of the relief valve unit in the plunger hole to communicate with outside of the plunger, the relief valve unit including a seat member, a valve ball housed in the seat member, a spring biasing the valve ball, a stopper member holding the spring and restricting a movement range of the valve ball, and a cap member fitted with the seat member and retaining the stopper member inside the seat member,
• the cap member having an oil leak part that discharges released oil to the relief-side space, and
• the external relief hole being opened radially in an outer circumferential surface of the plunger.
• wherein the oil leak part includes a lateral open part that discharges released oil directly in a radial direction.