An intake-side space around intake ports and an exhaust-side lower space around exhaust ports are provided in a main cooling jacket portion. These spaces are connected to each other. In a sub cooling jacket portion, an exhaust-side upper space is provided at a level above the exhaust-side lower space. The main cooling jacket portion and the sub cooling jacket portion are connected to each other via a cylindrical-hole connecting passage extending vertically and separate from each other vertically via a wall portion in another area than the connecting passage. Accordingly, a cooling device of an engine which can restrain the exhaust gas from being cooled too much improperly is provided.