The switching power source device obtains a desired DC voltage by controlling the current flowing through a coil by turning on and off a switching element by a PWM control. In the PWM ON period to turn on the switching element by the PWM control, the switching power source device is enabled to switch the switching element by a first pulse signal whose cycle is shorter than the PWM cycle and whose pulse width is gradually increased, in a first period just after the start of the PWM ON period. Further, the switching power source device is enabled to switch the switching element by a PWM signal based on the PWM control after the first period in the PWM ON period has elapsed. According to this approach, it is possible to reduce the harmonic noise.