Object A cooling water passage structure for an engine 50 is provided that can be simplified and improve the productivity of the engine 50.
Solving Means A cooling water passage structure includes an engine 50 having cylinder axes P arranged in a V-shape centering on a crankshaft 52; water jackets 62, 63 formed in front and rear cylinder blocks 53 F, 53R and cylinder heads 54, 54 of the engine 50; a thermostat case 71 having a portion formed integrally with the front and rear cylinder blocks 53F, 53R in the V-bank of the front and rear cylinder blocks 53F, 53R; a cooling water inlet side connecting portion 81 connecting a cooling water supply side line 65 with a cooling water passage including the water jackets 62, 63; and a cooling water outlet side connecting portion 82 connecting a cooling water discharge side line 66 with a cooling water passage including the water jackets 62, 63. The thermostat case 71, the cooling water inlet side connecting portion 81, and the cooling water outlet side connecting portion 82 are disposed at one end side of the engine 50 in the direction of the crankshaft 52.