An analog-to-digital converter that converts an analog input signal into a digital signal includes a comparator configured to compare a reference signal with an input signal and, if the input signal matches the reference signal, inverts an output; a counter configured to count a comparison time of the comparator; a control circuit configured to monitor the output of the comparator; a voltage generating circuit configured to generate, if a monitoring result obtained by the control circuit indicates that the output of the comparator is at a predetermined level, a direct current voltage in accordance with the monitoring result; and an analog adder configured to add the voltage generated by the voltage generating circuit to the input signal and supply a sum signal to an input terminal of the comparator.