A circuit for inhibiting a transition phenomenon in a power supply unit is disclosed. When the power supply mode is shifted, a transition phenomenon is made not to appear in the operating power supplied to the load. The circuit includes a microcomputer which judges as to the presence or absence of horizontal and vertical synchronizing signals supplied from the computer system so as to vary the power in a stepwise form when shifting the power supply mode. In accordance with control signals of the microcomputer, a feedback level adjusting section reduces the error detected voltage in a stepwise form when the power is fed back from a second rectifying section through an error detecting section to a pulse width modulator (PWM) control section. Therefore, in accordance with the stepwisely decreasing error detected voltage, the pulse width modulator (PWM) control section reduces its output signals in a stepwise form, so that the power supplied to the load can be reduced in a stepwise form.