There is provided a switching power source capable of quickly responding to a change in output at high efficiency over a wide output voltage range. The power source includes an integrator, a comparator which compares an output from the integrator with a quantization reference value, and a sampling circuit which samples an output from the comparator in synchronism with a clock signal. The power source further includes a .DELTA..SIGMA. modulator which quantizes an output from the integrator, outputs a quantized signal, and negative-feeds back the quantized signal to suppress the quantization error of the input signal. In the switching power source, a resistance value adjustment circuit changes the hysteresis width of the quantization reference value by changing the resistance value of the variable resistor of the comparator on the basis of a signal output from a detection circuit which detects at least one of an external control signal and the output voltage and load current of the switching power source.