A phase adjustment device comprising: 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, wherein a maximum sensitivity phase difference of one of the first and second detection signals does 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 wherein the detection signal generator comprises: a first branch unit configured to branch one of the two signals into first and second branch signals; a second branch unit configured to branch the other of the two signals into third and fourth branch signals; a first delayer (143; 243; 343; 543) configured to generate a first delayed signal obtained by delaying a phase of the first branch signal; a second delayer (144; 244; 344; 544) configured to generate a second delayed signal obtained by delaying a phase of the third branch signal; a first mixer (145; 445; 545) configured to generate a first synthesized signal obtained by synthesizing the first delayed signal and the fourth branch signal; a second mixer (146; 446; 546) configured to generate a second synthesized signal obtained by synthesizing the second delayed signal and the second branch signal; an adder configured to add the first synthesized signal and the second synthesized signal so as to generate the first detection signal; and a subtractor configured to subtract the second synthesized signal from the first synthesized signal so as to generate the second detection signal. 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, wherein a maximum sensitivity phase difference of one of the first and second detection signals does 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 wherein the detection signal generator comprises: a first branch unit configured to branch one of the two signals into first and second branch signals; a second branch unit configured to branch the other of the two signals into third and fourth branch signals; a first delayer (143; 243; 343; 543) configured to generate a first delayed signal obtained by delaying a phase of the first branch signal; a second delayer (144; 244; 344; 544) configured to generate a second delayed signal obtained by delaying a phase of the third branch signal; a first mixer (145; 445; 545) configured to generate a first synthesized signal obtained by synthesizing the first delayed signal and the fourth branch signal; a second mixer (146; 446; 546) configured to generate a second synthesized signal obtained by synthesizing the second delayed signal and the second branch signal; an adder configured to add the first synthesized signal and the second synthesized signal so as to generate the first detection signal; and a subtractor configured to subtract the second synthesized signal from the first synthesized signal so as to generate the second detection signal. a first branch unit configured to branch one of the two signals into first and second branch signals; a second branch unit configured to branch the other of the two signals into third and fourth branch signals; a first delayer (143; 243; 343; 543) configured to generate a first delayed signal obtained by delaying a phase of the first branch signal; a second delayer (144; 244; 344; 544) configured to generate a second delayed signal obtained by delaying a phase of the third branch signal; a first mixer (145; 445; 545) configured to generate a first synthesized signal obtained by synthesizing the first delayed signal and the fourth branch signal; a second mixer (146; 446; 546) configured to generate a second synthesized signal obtained by synthesizing the second delayed signal and the second branch signal; an adder configured to add the first synthesized signal and the second synthesized signal so as to generate the first detection signal; and a subtractor configured to subtract the second synthesized signal from the first synthesized signal so as to generate the second detection signal.