The circuit provides a type of switching power supply device that can reduce the ripple voltage generated in the output when switching is performed in the step-down/step-up control mode. Control part (CTRL) operates as follows: corresponding to the detection value of input potential detecting part 20, the step-down control mode/step-up control mode is switched; in each control mode, the clock signal generated by comparator 50 corresponding to output potential Vout is fed back to primary circuit (PRI)/secondary circuit (SEC); in this case, corresponding to the control mode, multiplexer 60 switches the input sign of transconductance amplifier 40 corresponding to the control mode.