A DLL includes a first variable delay circuit that variably delays a first transition of an external signal, a second variable delay circuit that variably delays a second transition of the external signal, a synthesis circuit that synthesizes output signals of the first variable delay circuit and the second variable delay circuit, a duty change detection circuit that changes and detects the duty of an output signal of the synthesis circuit, and delay control circuits that vary the delay of the first variable delay circuit or the second variable delay circuit in accordance with the result of duty detection by the duty change detection circuit.