A liquid cooled cooling device wherein occurrence of vibrations is controlled, the power for feeding water is lowered, and further, temperature variations around a heat generating section are prevented. The cooling device comprises: a jacket portion covering the outer peripheral portion of the heat generating section so as to surround it in order to modify the cooling liquid feeding construction and to modify the temperature conditions at flow-in and flow-out portions of the cooling liquid and the relationship between the temperature of the cooling water and the atmospheric temperature around the heat generating section and so forth, and provided at one end thereof with the flow-in portion and at the other with the flow-out portion, the said flow-in portion communicating with the flow-out portion; an insulating member covering the substantially entire peripheral portion of the jacket portion; and a temperature control means for setting the temperature of the cooling liquid flowing into the flow-in portion at a temperature lower than the atmospheric temperature in the space where the heat generating section is provided and for having the temperature of the cooling liquid flowing out of the flow-out portion coincide approximately with the atmospheric temperature.