A lidar 1 is provided with an APD 41 and a DSP 16. The APD 41 receives a return light Lr that is an outgoing light Lo radiated by a scanner 55 and thereafter reflected by an object. The DSP 16 includes a shot noise estimation unit 77 and a voltage control unit 78. The shot noise estimation unit 77 estimates, on the basis of the output signal from the APD 41, information associated with background light amount that is the sun light reflected by the object and thereafter received by the APD 41. The voltage control unit 78 determines, on the basis of the information associated with the estimated background light amount, the voltage to be applied to the APD 41 in order to control the gain M of the APD 41.