A projection image pickup device, comprising: a pulsed-light emitter; an optical sensor; a reference timing generator that generates a signal indicating an operation reference timing; a controller; and a signal processor, wherein during a first exposure period in a frame, the controller performs exposure control over the optical sensor in synchronization with the operation reference timing and causes the pulsed-light emitter to turn off, during a second exposure period in the frame, the controller causes the pulsed-light emitter to emit light in synchronization with the operation reference timing and performs exposure control over the optical sensor, during a third exposure period in the frame, the controller causes the pulsed-light emitter to emit light in synchronization with the operation reference timing and performs exposure control over the optical sensor in synchronization with a timing obtained by adding a predetermined delay time to the operation reference timing, the pulsed-light emitter emits light according to light emission control of the controller, the optical sensor is exposed according to the exposure control of the controller and outputs an output signal indicating an amount of exposure to the signal processor, and the signal processor: sets a differential signal between an output signal from the optical sensor in the second exposure period and an output signal from the optical sensor in the first exposure period as a first differential signal;, wherein the frame includes the first exposure period to an Nth exposure period, where N is an integer greater than or equal to 4, during a Kth exposure period in the frame, where K is an integer between 3 and N, both inclusive, the controller causes the pulsed-light emitter to emit light in synchronization with the operation reference timing in a (K-1)th exposure period and performs exposure control over the optical sensor, and the signal processor: sets a differential signal between an output signal from the optical sensor in the Kth exposure period and the output signal from the optical sensor in the first exposure period as a (K-1)th differential signal; and outputs a sum total of differential signals from the first differential signal to an (N-1)th differential signal, characterized by excluding any differential signal smaller than a predetermined threshold. a pulsed-light emitter; an optical sensor; a reference timing generator that generates a signal indicating an operation reference timing; a controller; and a signal processor, wherein during a first exposure period in a frame, the controller performs exposure control over the optical sensor in synchronization with the operation reference timing and causes the pulsed-light emitter to turn off, during a second exposure period in the frame, the controller causes the pulsed-light emitter to emit light in synchronization with the operation reference timing and performs exposure control over the optical sensor, during a third exposure period in the frame, the controller causes the pulsed-light emitter to emit light in synchronization with the operation reference timing and performs exposure control over the optical sensor in synchronization with a timing obtained by adding a predetermined delay time to the operation reference timing, the pulsed-light emitter emits light according to light emission control of the controller, the optical sensor is exposed according to the exposure control of the controller and outputs an output signal indicating an amount of exposure to the signal processor, and the signal processor: sets a differential signal between an output signal from the optical sensor in the second exposure period and an output signal from the optical sensor in the first exposure period as a first differential signal;, wherein sets a differential signal between an output signal from the optical sensor in the second exposure period and an output signal from the optical sensor in the first exposure period as a first differential signal;, wherein the frame includes the first exposure period to an Nth exposure period, where N is an integer greater than or equal to 4, during a Kth exposure period in the frame, where K is an integer between 3 and N, both inclusive, the controller causes the pulsed-light emitter to emit light in synchronization with the operation reference timing in a (K-1)th exposure period and performs exposure control over the optical sensor, and the signal processor: sets a differential signal between an output signal from the optical sensor in the Kth exposure period and the output signal from the optical sensor in the first exposure period as a (K-1)th differential signal; and outputs a sum total of differential signals from the first differential signal to an (N-1)th differential signal, characterized by excluding any differential signal smaller than a predetermined threshold. sets a differential signal between an output signal from the optical sensor in the Kth exposure period and the output signal from the optical sensor in the first exposure period as a (K-1)th differential signal; and outputs a sum total of differential signals from the first differential signal to an (N-1)th differential signal, characterized by excluding any differential signal smaller than a predetermined threshold.