PURPOSE: To prevent cooling water from running out by opening a vent space to ambient air when a vent conduit between the vent space between filling pipe flaps and a lid and an interior space is closed.

CONSTITUTION: Filling pipe flaps 4 protruding into a tank are arranged on a ceiling of an expansion tank 1, and regulate the liquid level with lower edges 5 thereof. A closed annular space 8 within a cylindrical wall 7 forms a vent conduit from the interior space of the tank to ambient air. A valve seat 13 is arranged in a hole 12 and constitutes a release valve with a ball 14. A lid 17 interrupts the vent conduit with respect to ambient air by means of a sealing ring 20. When the lid 17 is partially opened, pressurized air is released to pass through the hole 12 (the release valve seat 13), the annular space 8 and a hole 10 forming the vent conduit, through a vent space 22 to pass through a screw 16, to ambient air. Thus, it is possible to balance the pressure in the interior space 3 of the tank and the pressure of ambient air, and prevent cooling water from running out.