Each of plural sigma-delta modulators having a sampling capacitor, an integrator, and a quantizer are connected to each other in parallel. Each of the sigma-delta modulators conducts parallel oversampling in which an analog input signal is sampled by a sampling capacitor, and the sampling result is quantized by the integrator and the quantizer. Then, the quantized values of the sigma-delta modulators are added to obtain MSBs, the residue values of the integrators after quantizing in the respective sigma-delta modulators are added, and the addition result of the residue values is converted analog-to-digital to obtain LSBs.