An internal combustion engine includes a cam chain chamber accommodating a cam chain and a chain guide guiding the cam chain. The engine also includes a chain tensioner including a case main body, a plunger sliding in the case main body, a coil 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. The engine also includes oil supply passages in an inner wall of the cylinder head and an inner wall of the cylinder body and configured to supply oil to the chain tensioner.