In a power conversion module device of the present invention, a transformer, a primary power element and a secondary power element are mounted on a printed wiring board in surface-to-surface contact therewith, and no control circuit is mounted thereon. In addition to reducing impedance and loss in the wiring, this constitution improves the voltage waveform of the primary power element. Thus, noise reduction can be achieved. Further, because heat can be transferred to the printed wiring board, heat generated from the transformer, the primary power element and the secondary power element can be reduced. The size of the transformer can also be reduced.