A cooling apparatus includes a refrigerant tank having a surface to which heating devices are attached and containing liquid refrigerant which is boiled and evaporated by heat transferred from the heating devices, and a radiator for radiating the heat of the boiled and vaporized refrigerant. The radiator has an inflow return chamber provided with a small-diameter opening having a diameter smaller than that of the inflow return chamber. The liquid refrigerant of the gas-liquid mixed refrigerant is dammed by the small-diameter opening of the inflow return chamber and the dammed liquid refrigerant is returned through a return passage to the refrigerant tank. Since the liquid refrigerant contained in the gas-liquid mixed refrigerant is suppressed from flowing by means of the small-diameter opening, the gaseous refrigerant can transfer heat directly to the walls of the radiating passages, so that the deterioration of the radiating performance can be prevented.