To reduce noise generate by the film oscillation of a chain case.

In this engine chain case structure, the chain case of a chain is fastened to an engine body, the chain case includes a chain case fastening boss and an oil passage boss, and respective bosses are arranged below a pair of cam sprockets and across a cylinder axis. Connection between an annular rib at the outer periphery of a crankshaft insertion hole in the chain case 18 and the chain case fastening boss 23 is performed by a first reinforcing rib 38 extending along a chain inner periphery between a crankshaft and one camshaft. Connection between the longitudinal intermediate portion 39 of the first reinforcing rib and the oil passage boss is performed by a second reinforcing rib 40 extending along the chain inner periphery between the crankshaft and the other camshaft. Connection in a ladder shape between the outer periphery of the oil passage boss and the first reinforcing rib is performed by a plurality of third reinforcing ribs 42 respectively extending in an intersecting direction of the cylinder axis and having a clearance in a cylinder axis direction.