To improve the endurance of a hydraulic control valve by preventing a chain case from vibrating and a cylinder from being deformed in a chain case structure of an engine.

When viewing a chain case 8 from a direction of a crank shaft, lateral both sides of a mount bracket 13 are extended to one of left and right edges and upper edges 11 of the sides, and a fastening portion fastened to an engine body 2 is provided in the lower part of the mount bracket 13. Cylinders 18, 19 are arranged below the fastening portion with outer circumferential surfaces of the cylinders mutually coupled to the mount bracket 13. Below the mount bracket 13, there are arranged reinforcing ribs 26, 28 traversing the cylinders 18, 19 from the mount bracket 13 to extend downward.