According to one embodiment, a digital phase detector includes a chain of delay devices configured to receive a reference signal through a first stage and delay the reference signal at each stage. The phase detector includes a sampler group configured to include a first sampler samples a first signal of N-phase input signals (N is an integer of at least 2) in accordance with the reference signal and a second sampler samples a second signal of N-phase input signals which lags behind the first signal in phase by 2π/N in accordance with an output signal from the first stage of the chain of delay devices. The phase detector includes a detection circuit configured to detect a time difference between an edge of the reference signal and an edge of the first signal based on sampled signals from the sampler group to convert the time difference into a phase difference.