A cooling apparatus includes a refrigerant tank containing boiling and condensing refrigerant and a radiator for cooling the refrigerant. The radiator is connected to the refrigerant tank on a front surface thereof, and hot objects to be cooled are attached to a heat-receiving surface formed on a rear surface of the refrigerant tank. The refrigerant is circulated in the apparatus so that the refrigerant is vaporized in the refrigerant tank and condensed in the radiator. The radiator is connected to the refrigerant tank by inserting a pair of header tanks thereof into the refrigerant tank. The insertion length is controlled to obtain good heat conductivity between the radiator and the refrigerant tank and to avoid deformation of a radiator fin contacting the refrigerant tank. Holes for mounting a substrate carrying the hot objects may be made on enlarged portions of a cover plate closing the refrigerant tank. The heat-receiving surface may be elevated from the rear surface of the refrigerant tank to avoid interference between other components mounted on the substrate and the rear surface of the refrigerant tank.