PURPOSE: To improve the performance of all devices by largely setting the flowing sectional area of a cooling passage which is disposed at the center of a block larger than that of the passage which is disposed at the periphery, thereby cooling all the devices to uniform temperature.

CONSTITUTION: A flowing sectional area of cooling passage 6b for cooling a device which is disposed at the center of a block 1 which has a plurality of through holes is set larger than those of cooling passages 6a, 6c which are disposed at the periphery of the block. The quantity of heat dissipated into coolant from the respective devices 3 in the holes 2b, 2c becomes larger than that from the respective devices 3 into the coolant in the holes 2a, 2d. Then, large temperature difference is prevented from being produced the devices 3.