A cooler includes: a jacket having an internal coolant conduction space surrounded by a main cooling surface top plate, an opposite bottom plate, and a side wall; coolant inflow and outflow pipes connected to two through holes in the side wall; a coolant introduction channel forming a part of the coolant conduction space and communicating with the coolant inflow pipe; a coolant discharge channel forming a part of the coolant conduction space and communicating with the coolant outflow pipe; and a fin unit between the coolant introduction and discharge channels. The fin unit includes a plurality of fins having separate main surfaces and thermally connected to the top plate. The fins have first ends acutely angled relative to a direction of flow of coolant in the coolant introduction channel, and second ends acutely angled relative to a direction of flow of coolant in the coolant discharge channel.