A circuit assembly is disclosed which includes first and second substrates disposed on a heat dissipation base, and first and second semiconductor elements mounted on the first and second substrates. The first and second substrates are wired together, and three main electrode terminals are provided when the first and second semiconductor elements are connected in series, while two main electrode terminals are provided when the first and second semiconductor element are connected in parallel. In both cases, the circuit assembly is covered with a common exterior case so that one portion of each main electrode terminal or one portion of each main electrode terminal is exposed. Parts used in the circuit assembly are shared, and by changing the wiring between the first and second substrates, semiconductor modules with different functions are realized at low cost.