A cooling device for cooling a heat-generating member includes a refrigerant tank for boiling liquid refrigerant by heat from the heat-generating member and a radiator for cooling and condensing gas refrigerant from the refrigerant tank. The refrigerant tank has therein a plurality of refrigerant passages defined by ribs which are integrally formed with any one of opposite wall parts of the refrigerant tank. The ribs continuously extend in an up-down direction to be slightly longer than an up-down dimension of a boiling surface of the refrigerant tank. In the cooling device, because each passage width of the refrigerant passages is set to be equal to or smaller than double Laplace length, an outer diameter of bubbles in the refrigerant passages becomes larger, and liquid refrigerant can be moved upwardly by bubbles. Accordingly, it can restrict liquid refrigerant surface from being lowered even when gas-generating amount is increased.