A unit power conversion circuit comprising: a main circuit configured to include one or more capacitors and two or more main circuit switch elements (8, 9); and gate drive circuits driving the main circuit switch elements (8,9), characterized in the unit power conversion circuit being configured as a minimum circuit such that: the main circuit includes a high-sided main circuit switch element with a high-voltage terminal being connected to one terminal of the capacitor and a low-sided main circuit switch element with a low-voltage terminal being connected to the other terminal of the capacitor; the low-voltage terminal of the high-sided main circuit switch element and the high-voltage terminal of the low-sided main circuit switch element are respectively opened; and each gate drive circuit is insulated in signal and insulated in power source, alternatively, the unit power conversion circuits being configured as a circuit containing a plurality of the minimum circuits such that: the low-voltage terminal of one high-sided main circuit switch element is connected to the high-voltage terminal of the next high-sided main circuit switch element; the high-voltage terminal of one low-sided main circuit switch element is connected to the low-voltage terminal of the next low-sided main circuit switch element; the terminals are connected at multi-stages by repeating these connections twice or more; a lowest-voltage terminal of the high-sided main circuit switch element and a highest-voltage terminal of the low-sided main circuit switch element are respectively opened; and each gate drive circuit is insulated in signal and insulated in power source. a main circuit configured to include one or more capacitors and two or more main circuit switch elements (8, 9); and gate drive circuits driving the main circuit switch elements (8,9), characterized in the unit power conversion circuit being configured as a minimum circuit such that: the main circuit includes a high-sided main circuit switch element with a high-voltage terminal being connected to one terminal of the capacitor and a low-sided main circuit switch element with a low-voltage terminal being connected to the other terminal of the capacitor; the low-voltage terminal of the high-sided main circuit switch element and the high-voltage terminal of the low-sided main circuit switch element are respectively opened; and each gate drive circuit is insulated in signal and insulated in power source, alternatively, the main circuit includes a high-sided main circuit switch element with a high-voltage terminal being connected to one terminal of the capacitor and a low-sided main circuit switch element with a low-voltage terminal being connected to the other terminal of the capacitor; the low-voltage terminal of the high-sided main circuit switch element and the high-voltage terminal of the low-sided main circuit switch element are respectively opened; and each gate drive circuit is insulated in signal and insulated in power source, alternatively, the unit power conversion circuits being configured as a circuit containing a plurality of the minimum circuits such that: the low-voltage terminal of one high-sided main circuit switch element is connected to the high-voltage terminal of the next high-sided main circuit switch element; the high-voltage terminal of one low-sided main circuit switch element is connected to the low-voltage terminal of the next low-sided main circuit switch element; the terminals are connected at multi-stages by repeating these connections twice or more; a lowest-voltage terminal of the high-sided main circuit switch element and a highest-voltage terminal of the low-sided main circuit switch element are respectively opened; and each gate drive circuit is insulated in signal and insulated in power source. the low-voltage terminal of one high-sided main circuit switch element is connected to the high-voltage terminal of the next high-sided main circuit switch element; the high-voltage terminal of one low-sided main circuit switch element is connected to the low-voltage terminal of the next low-sided main circuit switch element; the terminals are connected at multi-stages by repeating these connections twice or more; a lowest-voltage terminal of the high-sided main circuit switch element and a highest-voltage terminal of the low-sided main circuit switch element are respectively opened; and each gate drive circuit is insulated in signal and insulated in power source.