A position detector, comprising: a scale comprising a pattern array formed cyclically in a movement direction, the scale being provided to one of a fixed member and a movable member movable relative to each other in the movement direction; a detector comprising a plurality of detection elements arranged in the movement direction, the detector being provided to the other of the fixed member and the movable member; a generating unit configured to selectively generate one of a first signal obtained by a first combination of signals received from the plurality of detection elements and a second signal obtained by a second combination of signals received from the plurality of detection elements; an output unit configured to output one of the first signal and the second signal from the generating unit, a processing unit configured to process the output first signal and the output second signal and to respectively obtain information of a relative movement between the scale and the detector and information of an abnormality of the pattern array; and a switcher configured to switch a signal to be generated by the generating unit between the first signal and the second signal, wherein the first and second combination of signals is such that: the first signal (A1, B1) periodically changes with the relative movement between the scale and the detector; and the second signal (A2, B2) does not periodically change with the relative movement between the scale and the detector, the second combination subtracting signals among the signals received from the plurality of detection elements from each other which differ in phase by 2nn, such that the second signal falls within a threshold range, in a case where the abnormality does not exist, and falls outside the threshold range, in a case where the abnormality does exist. a scale comprising a pattern array formed cyclically in a movement direction, the scale being provided to one of a fixed member and a movable member movable relative to each other in the movement direction; a detector comprising a plurality of detection elements arranged in the movement direction, the detector being provided to the other of the fixed member and the movable member; a generating unit configured to selectively generate one of a first signal obtained by a first combination of signals received from the plurality of detection elements and a second signal obtained by a second combination of signals received from the plurality of detection elements; an output unit configured to output one of the first signal and the second signal from the generating unit, a processing unit configured to process the output first signal and the output second signal and to respectively obtain information of a relative movement between the scale and the detector and information of an abnormality of the pattern array; and a switcher configured to switch a signal to be generated by the generating unit between the first signal and the second signal, wherein the first and second combination of signals is such that: the first signal (A1, B1) periodically changes with the relative movement between the scale and the detector; and the second signal (A2, B2) does not periodically change with the relative movement between the scale and the detector, the second combination subtracting signals among the signals received from the plurality of detection elements from each other which differ in phase by 2nn, such that the second signal falls within a threshold range, in a case where the abnormality does not exist, and falls outside the threshold range, in a case where the abnormality does exist. the first signal (A1, B1) periodically changes with the relative movement between the scale and the detector; and the second signal (A2, B2) does not periodically change with the relative movement between the scale and the detector, the second combination subtracting signals among the signals received from the plurality of detection elements from each other which differ in phase by 2nn, such that the second signal falls within a threshold range, in a case where the abnormality does not exist, and falls outside the threshold range, in a case where the abnormality does exist.