A switching power supply apparatus includes a step-down converter that steps down an input current by turning on/off switching elements, a switching element electrically connected between the step-down converter and voltage outputs, and a microcontroller. An input-side resistor voltage dividing circuit and an output-side resistor voltage dividing circuit including elements with the same or similar specifications are electrically connected to the input and output of the switching element, respectively. The microcontroller stores coefficients of an expression which have been externally calculated according to a voltage detected by the output-side resistor voltage dividing circuit and a voltage detected at the voltage output portions. The microcontroller corrects a measurement value of an intermediate bus voltage detected by the input-side resistor voltage dividing circuit according to an expression and the stored coefficients.