A chain case structure of an engine, wherein, when a chain case is viewed in the crankshaft direction, both side portions of a mount bracket unit in the width direction extend to the left edge portion or the right edge portion of the chain case and the upper edge portion of the chain case, and a fastening portion to an engine main body is provided on the lower portion of the mount bracket portion. A cylinder portion of a hydraulic control valve of a variable valve mechanism is disposed below the fastening portion while the outer peripheral surface of the cylinder portion is mutually coupled to the mount bracket portion. Reinforcement ribs extend downward from the bottom of the mount bracket, across the cylinder portion.