A cooling structure for an outboard engine having a cooling water piping of reduced length and simplified construction. In the outboard engine that cools its engine with cooling water to be led from the outside, a water check port is provided in the middle of its cooling water passage. Moreover, a vapor separator is provided on a fuel supply equipment. In addition, a water passage for the water check port is provided so as to branch in the middle of the cooling water passage for a vapor separator. As such, the cooling water is led to the vapor separator and the water check port is located at the inlet of the cooling water before cooling of the engine.