One surface of a base section having an open portion forming an inlet port of a fluid is thermally connected to a target module to be cooled. Pluralities of fins arranged in parallel are mounted on the other surface of a base section in a direction substantially perpendicular to the base section. A fan is arranged to permit the fluid to flow through the clearance between the adjacent fins. A wall section open to the inlet port of the fluid is mounted on the base section. A part of the wall section constitutes a detachable lid section. A partition plate having through-holes formed therein is arranged between the base section and the lid section so as to divide the space between the base section and the lid section into two fluid flowing channels consisting of a main flowing channel and an auxiliary flowing channel.