PURPOSE: To prevent unbalance of current among elements when semiconductor units are connected between bus bars in series, by constructing each of the bus bars so as to have different thicknesses or widths of a specific ratio, the difference of the thicknesses or widths being represented by arithmetic progression.

CONSTITUTION: A bus bar 12A to which terminals 5A1, 5B1 and 5C1 are connected has a larger thickness T1 on the lefthand side close to the arrow IN and a smaller thickness T2 on the righthand side, as viewed on the drawing. Ratio of the thickness T1 to the thickness T2 is equal to a ratio of a number of semiconductor elements 1A through 1C connected in series-1:1, and T1=2T2 in this case. The boundary of the section having the thickness T1 and the section having T2 corresponds to the righthand end of a terminal plate 5B2 located at the center, while terminal plates are spaced by an equal distance l. Similarly, a bus bar 12B to which terminal plates 5A2, 5B2 and 5C2 are connected has similar configurations to those of the bus bar 12A while the terminal plates are spaced by the same distance l as those on the bus bar 12A. According to such arrangement, current paths in the elements 1A through 1C have equal resistances and, therefore, unbalance of current among the elements due to voltage drop in the bus bars 12A, 12B can be prevented effectively.