PURPOSE: To obtain an inverter which has both the benefit of an inverter serving also as a step-up/step-down apparatus and the benefit of an inverter serving also as a step-up apparatus by a method wherein the operations of switching devices are changed in accordance with the comparison result between a load circuit voltage and a power supply voltage to select one of a step-up/step-down mode and a step-up mode.

CONSTITUTION: A capacitor C is connected in parallel to a series circuit of a DC power supply Vs and an autotransformer T and to a series circuit of an inductor L and a load Z to compose a load circuit. Switching devices S1, S2, S3 and S4 are controlled to operate by a controller 1. If a load voltage Vz is lower than a power supply voltage Vs, a step-up/step-down mode is selected to facilitate the output of the load voltage Vz in a range which can not be controlled with a step-up mode. If the load voltage Vz is higher than the power supply voltage Vs, the step-up mode is selected to facilitate efficient power supply operation. Moreover, if the load voltage Vz is significantly highter than the power supply voltage Vs, a mode having better characteristics is selected among the step-up/step-down mode and the step-up mode.