An A/D converter compares one or more analog voltages to be converted with a reference voltage given by a voltage change value of ramp voltage whose voltage value changes monotonically for a certain period or a voltage proportional to the voltage change value, converts each analog voltage to a digital value corresponding to the reference voltage, and outputs it, the A/D converter comprising an arithmetic unit for comparison between the analog voltage and reference voltage with respect to each analog voltage, the arithmetic unit having a first power supply line for receiving a power supply voltage, wherein the first power supply line is provided as another power supply line not affected by voltage fluctuation of a second power supply line for supplying a system power supply voltage by providing a MOS transistor whose gate terminal is connected to a stabilized voltage source between the first and second power supply lines.