An analog-to-digital converter converting an analog input signal into a digital signal includes a comparator comparing a reference signal with an input signal and, if the reference signal matches the input signal, inverting an output; and a counter counting a comparison time. The counter includes flip flops that perform serial input/output. An input and an output of the counter are interconnected. The counter operates in a counter mode and a shift register mode. In the counter mode, a data output of each flip flop is supplied to a clock input of the next flip flop, and, if the output of the comparator is at a predetermined level, the counter functions as a counter synchronized with a counter clock signal. In the shift register mode, the flip flops are cascade-connected, and the counter functions as a shift register synchronized with a shift register clock signal.