logical operation circuit 32 at a high level during the time when the level of the modulating signal e 1 is higher than that of the carriers f 1 and g 1 and at a low level during the period when the level of the modulating signal is lower than that of the carriers logical operation circuit 32 forms the signals k 1 l 1 m 1 and n 1 out of these reference on off signals i 1 and j 1 and a square wave h 1 which corresponds to the sine wave or the output of the sine wave generator 26 to control the on off of the switch elements 3 1 3 4 fig5 according to the logical expression below the signals k 1 n 1 are coupled respectively to switch elements 3 1 3 4 to drive a converter circuit 2 such as shown in fig5 prior art thereby forming the pulse width modulated ac output voltage o 1 having three levels of voltages e o and e as described in the foregoing this embodiment is characterized in that the output voltage a 1 of the inverter is compared with the sine wave d 1 which is amplitude.