A phase adjustment device includes: a detection signal generator configured to generate a pair of first and second detection signals for detecting a phase difference between two signals whose phases have been adjusted by two phase adjusters, respectively, a maximum sensitivity phase difference of one of the first and second detection signals being not overlap with that of the other, and detection sensitivity of the phase difference becoming maximum at the maximum sensitivity phase difference; a detection signal selector configured to select one of the first and second detection signals whose predetermined range around the maximum sensitivity phase difference covers a preset phase difference; and a phase controller configured to control an amount of phase-adjusting by at least one of the two phase adjusters based on a difference between the phase difference detected within the predetermined range using the selected detection signal and the preset phase difference.