To present a control circuit of a DC--DC converter to be incorporated in a logic circuit, having a digital error amplifier capable of controlling the gain depending on an input voltage and an output voltage, without requiring a resistance or a capacitor of high precision used in a feedback circuit or the like. A .SIGMA..DELTA. AD converter type error amplifier 10 includes an arithmetic unit 20, an integrating unit 21, a 1-bit quantizing unit 22, a D/A converter 23, and a first counter 24. The arithmetic unit 20 outputs a differential signal of an output voltage Vout and an average output voltage AV. The integrating unit 21 outputs an integral signal by integrating the differential signal. The 1-bit quantizing unit 22 outputs a 1-bit digital signal by quantizing the integral signal. The D/A converter 23 converts from digital to analog depending on a 1-bit digital signal. A digital PWM circuit 11 determines the on-duty of the main switching element of the DC--DC converter depending on the pulse density of the D/A converter 23.