PURPOSE: To control the output AC current at high speed so as to follow a set AC current even when the switching frequency of a self-excitated semiconductor power converter is not high and to make the conversion apparatus resistant to a disturbance.

CONSTITUTION: This conversion apparatus is constituted of a feed-forward power control circuit 302 which generates the output voltage instruction of a semiconductor power converter from the phase and the amplitude of a set AC current, of a power control circuit 303 which corrects the output voltage instruction of the feed-forward control circuit 302 by the deviation between a fed-back system current and the set AC current, of a transformer 33 which defects a system voltage and of a power conversion circuit 304 which converts the output voltage of the semiconductor power converter as the sum of output voltages obtained from it so as to be output from the semiconductor power converter 31.