An electronic device capable of preventing air from entering a pump for circulating a cooling medium is provided in a small-sized electronic computer provided with a liquid cooling system, which has a liquid cooling construction for circulating a cooling liquid with the use of a power source and for cooling a high temperature part of with the cooling liquid. The electronic device of the invention comprises a gas entering preventing mechanism provided in a part of a circulating path, through which the cooling liquid circulates, to prevent air from entering the power source.