In a liquid cooling jacket, to improve the heat transfer coefficient, extensibility and assembling, the liquid cooling jacket comprises a base bonded to a heating element; a post standing perpendicularly to the base; a plurality of radiating fins attached to the post and arranged so as to be parallel to the base; a partition filling up intervals between the plurality of radiating fins at a predetermined width; and a case which surrounds the post and the radiating fins and is bonded to the base, and to which an inlet and an outlet for coolant are attached at positions where flow of the coolant is divided by the partition. The plurality of radiating fins may be arranged at intervals each of which is narrow in comparison with a thickness of each of the plurality of radiating fins. Therefore, because the coolant flow within the liquid cooling jacket ensures a plurality of passages, the passage resistance is low. In addition, by setting the size of each of the inlet and outlet for the coolant to be substantially equal to the height of the arranged radiating fins, the flow rates on the radiating fins can be even.