PURPOSE: To cool all devices in uniform temperature distribution by setting the sectional areas of the cooling passages to correspond to the heat generation distribution of the respective devices.

CONSTITUTION: The sectional areas of cooling passages 6 which are provided along the holes 2 for containing the devices 3 of the block 1 are set to correspond to the heat generation distribution of the devices 3. For example, when the devices 3 have heat generation sources at the upper position, the sectional area of the upper part of the passage 6 is set larger than that of the lower part. Then, the heat sink amount to the coolant in the passage 6 from the upper part of the device 3 becomes larger than that from the lower part, and the entire devices can be cooled so that the uniform temperature distribution is obtained over the entire devices.