An optical scanning device comprising: a light-source device to emit light to scan a scanning range; an optical scanning system disposed on an optical path of the light emitted from the light-source device ; a first light detector (D1) including a first light-receiving element (PD1); a second light detector (D2) including a second light-receiving element (PD2), the first light-receiving element (PD1) and the second light-receiving element (PD2) disposed side by side along a main scanning direction in the scanning range and configured to respectively output a first detection signal and a second detection signal; and a comparator (COMP) configured to compare the first detection signal of the first light detector (D1) and the second detection signal of the second light detector (D2) and output a comparative signal; characterised in that the optical scanning device further comprises: a control system configured to set a target value (Pt) of a light-emission power (P) of the light-source device based on a level Ps of the light-emission power (P) at which an output time (T) has a minimum value, the light-emission power (P) having the target value (Pt) to be used for scanning the first light-receiving element (PD1) and the second light-receiving element (PD2), and wherein the control system is configured to determine a function (P-T) of variations in the output time (T) with the light emission power (P), the output time (T) being a time period from a reference time point to a time at which the comparator (COMP) outputs the comparative signal. a light-source device to emit light to scan a scanning range; an optical scanning system disposed on an optical path of the light emitted from the light-source device ; a first light detector (D1) including a first light-receiving element (PD1); a second light detector (D2) including a second light-receiving element (PD2), the first light-receiving element (PD1) and the second light-receiving element (PD2) disposed side by side along a main scanning direction in the scanning range and configured to respectively output a first detection signal and a second detection signal; and a comparator (COMP) configured to compare the first detection signal of the first light detector (D1) and the second detection signal of the second light detector (D2) and output a comparative signal; characterised in that the optical scanning device further comprises: a control system configured to set a target value (Pt) of a light-emission power (P) of the light-source device based on a level Ps of the light-emission power (P) at which an output time (T) has a minimum value, the light-emission power (P) having the target value (Pt) to be used for scanning the first light-receiving element (PD1) and the second light-receiving element (PD2), and wherein the control system is configured to determine a function (P-T) of variations in the output time (T) with the light emission power (P), the output time (T) being a time period from a reference time point to a time at which the comparator (COMP) outputs the comparative signal. a control system configured to set a target value (Pt) of a light-emission power (P) of the light-source device based on a level Ps of the light-emission power (P) at which an output time (T) has a minimum value, the light-emission power (P) having the target value (Pt) to be used for scanning the first light-receiving element (PD1) and the second light-receiving element (PD2), and wherein the control system is configured to determine a function (P-T) of variations in the output time (T) with the light emission power (P), the output time (T) being a time period from a reference time point to a time at which the comparator (COMP) outputs the comparative signal. <EOS>