Reserve tank

A reserve tank disposed in a passage through which a cooling water circulates, the reserve tank comprising:

• a vapor-liquid separator that defines an inner space for separating a vapor phase and a liquid phase of the cooling water,
• an inlet portion that defines a first opening through which the cooling water is supplied into the inner space,
• an outlet portion that defines a second opening through which the cooling water is discharged from the inner space, and
• a protrusion that protrudes upward from a bottom surface of the vapor-liquid separator, wherein
• an inner circumferential surface of the vapor-liquid separator and an outer circumferential surface of the protrusion are coaxially provided in a cross-section taken along a horizontal surface to define an annular passage between the inner circumferential surface and the outer circumferential surface in the inner space, and
• the cooling water introduced from the first opening flows spirally in the annular passage.