PURPOSE: To efficiently separate oil through the centrifugal action of a sucked coolant, by conducting the sucked coolant into the oil separation chamber of a rear cylinder head so that the coolant revolves.

CONSTITUTION: A sucked coolant conducted to a suction port 33 enters into an oil separation chamber 23 so that the coolant is sucked into a suction hole 24b while revolved by the guide wall 24c of the separation chamber 23. The coolant is then conducted into a low pressure chamber 18b. At that time, oil contained in the coolant clings to a cylinder head 12b, the guide wall 24c, etc. because of centrifugal action due to the revolution. The clinging oil is moved in the flowing direction of the coolant and collected into an oil gathering chamber 35 through an oil inflow slit 34. The collected oil is recovered to an oil reservoir 6 through a passage 36.