According to an embodiment, an analog-to-digital converter includes a voltage generating unit to generate comparative voltages; and comparators. Each comparator compares any one of the comparative voltages with an analog input voltage and output a digital signal. Each comparator includes a differential pair circuit to detect a potential difference between two inputs. The differential pair circuit includes first and second circuit portions. The first circuit portion includes a first transistor having a gate to which one input is supplied; and a resistor connected in series with the first transistor. The second circuit portion includes a second transistor having a gate to which the other input is supplied and forms a differential pair with the first transistor; and a variable resistor connected in series with the second transistor. The variable resistor includes variable resistive elements each having a resistance value variably set according to a control signal.