PURPOSE: To reliably prevent large deflection of a cam chain at the same time when minute chain vibration of a cam chain is prevented from being transmitted to a cylinder block.

CONSTITUTION: A chain guide 14 made of synthetic resin is arranged on the stretch side of a cam chain 7. The chain guide 14 comprises a cover part 15 with which the front of the cam chain 7 is covered; a first guide wall 16 extending along the outer peripheral side of the cam chain 7: and a second guide wall 21 extending along the inner peripheral side thereof, and the cover part 15 is fixed to a cylinder block 2 by means of a bolt 26. The first guide wall 16 is extended in an L-shape in cross section from the cover part 15. The back 16a of the first guide wall 16 is positioned facing a wall surface 9a on the flange part 9 side of a cylinder block 2 being a support surface with a minute gap α therebetween.