Tensioner and relief valve unit

A tensioner comprising:

• a tensioner body having a cylindrical plunger bore with an open end;
• a cylindrical plunger slidably inserted in the cylindrical plunger bore;
• a biasing means that is accommodated inside an oil pressure chamber formed inside the cylindrical plunger such as to freely expand and contract, and that biases the cylindrical plunger outward;
• a check valve unit that stops reverse flow of oil flowing into the oil pressure chamber;
• and
• a relief valve unit that releases oil when pressure inside the oil pressure chamber reaches or exceeds a predetermined high pressure,
• the check valve unit being disposed as a valve element of the relief valve unit,
• the relief valve unit including a relief sleeve in which the check valve unit is slidably inserted, a relief valve seat that opens and closes as the check valve unit slides, and a unit pressing spring that presses the check valve unit toward the relief valve seat, and
• the check valve unit including an oil passage for allowing oil to flow from an outer side to an inner side of the check valve unit,
• wherein the oil passage includes an oil passage groove formed radially in an end face of the check valve unit on the unit pressing spring side.