A jitter control circuit, which reduces conducted EMI noise by giving jitter (frequency diffusion) to the operating frequency for driving a switching element, determines an operating frequency fc (e.g., 40 kHz) at which reduction effects change from a feedback voltage that represents magnitude of a load. The jitter control circuit causes a, for example, 8-bit counter that generates a modulation frequency to operate with, for example, 8 bits when 40 kHz≦fc and 7 bits when fc