To include a phase determining circuit that generates a first phase determination signal, a sampling circuit that samples the first phase determination signal and generates a second phase determination signal based on the sampled first phase determination signal, and a clock generating unit that generates an internal clock signal based on the second phase determination signal. The sampling circuit includes a continuity determining circuit that fixes the second phase determination signal when a logic level of the first phase determination signal changes within a sampling cycle, an initial operation circuit that fixes the second phase determination signal at a high level until when a third phase determination signal indicates a high level, and a disabling circuit that disables an operation of the continuity determining circuit after the third phase determination signal indicates a high level.