PURPOSE: To prevent worsening of sound/vibration property through improvement of rigidity of the periphery of an opening part by a method wherein a working window through which a water pump unit is inputted and outputted is opened in a front cover by which a chain chamber is partitioned to the front side of an engine block and a guide rib is formed on an opening edge part.

CONSTITUTION: A chain 12 to transmit rotation of a crank shaft 8 to two cam shafts on the intake side and the exhaust side is disposed on the front sides of engine blocks 2-4. Further, a front cover 5 to partition a chain chamber 6 through which a chain 12 is circulated is provided. A unit 32 consisting mainly of a water pump 1 to effect the forced feed of cooling water is disposed to a chain chamber 6. In this constitution, a working window 51 through which a water pump unit 32 is inputted and outputted is opened to the front cover 5. A guide rib 56 connected to the opening edge 55 of the working window 51 and protruded toward the inside of the chain chamber 6 is formed. This constitution improves rigidity of the periphery of the opening edge part 55.