To provide a small and light power conversion system having a small number of parts and improved reliability in system.

In a cooling fan of a cooling unit for a semiconductor element 1, the semiconductor element 1 is mounted on one face of a cooling block 2, and a plurality of cooling fins are mounted on the other face of the cooling block 2. In this case, the cooling fins are projected to the outside, through the hole of a main frame body 3, and the power conversion system is made waterproof, by using a waterproof packing at an interface part between the cooling block 2 and the main frame body 3. A packing guide 8 for supporting the waterproof packing 7 is mounted directly on the cooling block 2.