current and switch pulses as they change with a lapse of time are shown in fig1 in fig1 a reference letter a represents a waveform illustrating a time series of change in value of voltage induced in the primary coil 322 of the transformer 321 taken at a point u201c o u201d in the power supply circuit of fig1 a letter b a waveform illustrating a time series of change in amount of current in the point u201c o u201d of the power supply circuit and a letter c a waveform illustrating a time series of change in shape of an output wave of the control circuit 327 fed to the first switching element 324 during a period of a to b when a pulse wave the output wave of a predetermined duration shown by the waveform c is input from the control circuit 327 to the first switching element 324 making the first switching element 324 into an on state amount of electric current in the point u201c o u201d increases with time in proportion to a duration of the on state of the first switching element 324 as shown by the waveform b and thereby energy is charged into the primary coil 322 during a period of b to c.