In a heat storage tank (100), first and second valve bodies (119, 120) are disposed to open and close first and second valve ports (119a, 120a), respectively. A density of the first valve body is made larger than that of cooling water, and a density of the second valve body is made smaller than that of cooling water. When the dynamical pressure due to cooling water is applied to the first and second valve bodies, the first and second valve ports are opened by the first and second valve bodies. On the other hand, when the dynamical pressure is not applied to the first and second valve bodies, the first valve body closes the first valve port by the weight of the first valve body, and the second valve body closes the second valve port by the buoyancy.