Provided is an in-vehicle step-down switching power supply capable of obtaining a stable output while suppressing nose in a harmful frequency band even when an input voltage decreases. The in-vehicle step-down switching power supply includes a switching frequency control unit and the switching frequency control unit includes a switching frequency range setting unit that changes a switching frequency between fa and fb and a switching frequency path setting unit that skips a frequency band from fd to fe between fa and fb, via an intermediate frequency fc between fa and fb.