A voltage resonant converter is provided with a secondary side parallel resonant circuit, and an isolated converter transformer PIT is set in a state of loose coupling at a coupling coefficient k=about 0.7 or lower. Thus a steep unimodal characteristic is obtained as a constant-voltage control characteristic to reduce a control range of switching frequency which range is necessary for stabilization. A primary side parallel resonance frequency fo1 and a secondary side parallel resonance frequency fo2 are set so as to obtain favorable power conversion efficiency characteristics.