Abstract: An advanced liquid cooling system with an advanced cooling characteristic and high reliability, which is small in size and light in weight, is provided. In the liquid cooling system, heating units 2, which are semiconductors, are fixed on a base having high thermal conductivity. A liquid-passage hole 7 in the base 5 allows a liquid refrigerant 6 to pass through. Since the base 5 is made of material with high thermal conductivity, and the liquid-passage hole 7 extends in the vicinity of the heating unit 2 in the base, it is sufficient to transfer the heat of heating unit 2 the liquid refrigerant 6. This enables control of the temperature rise of the heating unit to be maintained low.