To prevent troubles when performing soft start by using a PWM signal generating device for a power supply device.

Immediately after a PWM signal generating device starts operating, the output value of a DFF 5 is set to 'OH' and a signal EN1 is set to 'L'. The output of a gate 612 becomes 'H' by the output value 'OH' and 'L' of a signal PM1ONS, the output 'OH' of the DFF 5 is changed to '1H' and sent to a CB 15 and an operation similar to the case when an on-width is '1H' is performed within the device. Although a signal PM1 changes from 'L' to 'H' but the signal EN1 is 'L' so that a signal PWM1 maintains 'L' by a gate. The output value of the DFF 5 is inputted to the CB15 while it stays 'OH' by the 'L' of a signal RDWN 1. Then, the register value of the DFF 5 is updated to '1H' through a specific operation, the signal EN1 of an on-width output control part 61 also changes to 'H', and a PWM signal is outputted.