PURPOSE: To obtain the pulse train of a duty close to the average value of a sine wave in each block even when considerably increasing the number of divided stepwise waves in the block.

CONSTITUTION: Signals from an oscillation circuit 6 are supplied to a rectangular wave generating circuit 7. The rectangular wave generating circuit 7 prepares the rectangular wave of a cycle Tm from the signals from the oscillation circuit 6, and these rectangular waves are supplied to a digital filter circuit 8 as digital data. Digital data expressing stepwise waves are prepared from the rectangular waves from the rectangular wave generating circuit 7 by the digital filter circuit 8 and supplied to an amplitude/pulse train transformation circuit 9. The amplitude/pulse train transformation circuit 9 generates the pulse train so that the output of the digital filter circuit 8 can be equal to the rate of generating high pulses. Thus, even when the number of divided stepwise waves in the block is considerably increased, the pulse train of the duty close to the average value of the sine wave can be obtained in each block.