An object detector comprising: a plurality of light source systems (11, 12) each including a light emitter to emit light; an optical system (20, 30) disposed in an optical path of the light emitted from the light emitters; a plurality of light detection systems (41, 42, 44) each including a light receiver to receive the light reflected or scattered from an object in a projection range; and a control system to detect information regarding the object based on the light received by the light detection system (41, 42, 44); wherein: the optical system is configured to generate: a first optical path to guide first light rays of the light to the projection range; and a second optical path to guide second light rays of the light emitted from a light emitter of each of at least two of the plurality of light source systems to a light receiver of each of at least two of the plurality of light detection systems (42, 41, 44) without guiding the second light rays to the projection range, the light detection systems (41, 42, 44) are configured to output a signal according to an amount of light received by the respective light receiver, and the control system is configured to detect conditions of the light source systems (11, 12) and the light detection systems (41, 42, 44) based on the second optical path light rays and on at least two signals output from the at least two light detection systems. a plurality of light source systems (11, 12) each including a light emitter to emit light; an optical system (20, 30) disposed in an optical path of the light emitted from the light emitters; a plurality of light detection systems (41, 42, 44) each including a light receiver to receive the light reflected or scattered from an object in a projection range; and a control system to detect information regarding the object based on the light received by the light detection system (41, 42, 44); wherein: the optical system is configured to generate: a first optical path to guide first light rays of the light to the projection range; and a second optical path to guide second light rays of the light emitted from a light emitter of each of at least two of the plurality of light source systems to a light receiver of each of at least two of the plurality of light detection systems (42, 41, 44) without guiding the second light rays to the projection range, the light detection systems (41, 42, 44) are configured to output a signal according to an amount of light received by the respective light receiver, and the control system is configured to detect conditions of the light source systems (11, 12) and the light detection systems (41, 42, 44) based on the second optical path light rays and on at least two signals output from the at least two light detection systems. the optical system is configured to generate: a first optical path to guide first light rays of the light to the projection range; and a second optical path to guide second light rays of the light emitted from a light emitter of each of at least two of the plurality of light source systems to a light receiver of each of at least two of the plurality of light detection systems (42, 41, 44) without guiding the second light rays to the projection range, a first optical path to guide first light rays of the light to the projection range; and a second optical path to guide second light rays of the light emitted from a light emitter of each of at least two of the plurality of light source systems to a light receiver of each of at least two of the plurality of light detection systems (42, 41, 44) without guiding the second light rays to the projection range, the light detection systems (41, 42, 44) are configured to output a signal according to an amount of light received by the respective light receiver, and the control system is configured to detect conditions of the light source systems (11, 12) and the light detection systems (41, 42, 44) based on the second optical path light rays and on at least two signals output from the at least two light detection systems.