Internal combustion engine and motorcycle equipped with the engine

An internal combustion engine comprising:

• a cylinder body including a cylinder therein;
• a cylinder head joined to the cylinder body;
• a cam chain chamber provided in the cylinder body and the cylinder head, and configured to accommodate a cam chain and a chain guide configured to guide the cam chain;
• a chain tensioner provided in the cam chain chamber, and including a case main body, a plunger sliding in the case main body, a spring biasing the chain guide against the cam chain via the plunger, interlocking teeth located on a surface of the plunger, and an engaging portion engaged with the interlocking teeth and configured to inhibit the plunger from moving in a direction away from the chain guide;
• and
• an oil supply passage provided in an inner wall of at least one of the cylinder head and the cylinder body, and configured to supply oil to the chain tensioner.