PURPOSE: To increase the heat radiation area and shorten the length by sliding the diode mounting seat faces at the middle positions upward thereby making upper and lower heat sinks in a pair into stepped ones, and providing both heat sinks with many cooling fins each, and forming the middle sections in convex shape.

CONSTITUTION: Heat sinks 41 and 42 are made into stepped ones, with the middle positions of the seats for mounting diodes 25 and 26 being slid upward from both ends, and both sides are opposed up and down. Each heat sink 41 and 42 is provided with many cooling fins 41a and 42a. The top side of the upper cooling fins 41a is made in convex shape and the top side of the lower cooling fins 42a in concave shape, thus the cooling fins 41a and 42a at the middle positions, which require heat radiation and cooling most, are made high. Hereby, the heat radiation area increases, and the cooling efficiency can be improved, and the lengths L of the heat sinks 41 and 42 can be shortened.