An optical scanning device comprising: a light-source device to emit light to scan a scanning range; an optical scanning system (11, 15) disposed on an optical path of the light emitted from the light-source device; a first light detector including a first light-receiving element; a second light detector including a second light-receiving element, the first light-receiving element and the second light-receiving element 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 and the second detection signal of the second light detector 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 of the light-source device based on a level Ps of the light-emission power at which an output time has a minimum value, the light-emission power having the target value (Pt) to be used for scanning the first light-receiving element and the second light-receiving element, and wherein the control system is configured to determine a function (P-T) of variations in the output time with the light emission power, the output time 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 (11, 15) disposed on an optical path of the light emitted from the light-source device; a first light detector including a first light-receiving element; a second light detector including a second light-receiving element, the first light-receiving element and the second light-receiving element 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 and the second detection signal of the second light detector 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 of the light-source device based on a level Ps of the light-emission power at which an output time has a minimum value, the light-emission power having the target value (Pt) to be used for scanning the first light-receiving element and the second light-receiving element, and wherein the control system is configured to determine a function (P-T) of variations in the output time with the light emission power, the output time 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 of the light-source device based on a level Ps of the light-emission power at which an output time has a minimum value, the light-emission power having the target value (Pt) to be used for scanning the first light-receiving element and the second light-receiving element, and wherein the control system is configured to determine a function (P-T) of variations in the output time with the light emission power, the output time being a time period from a reference time point to a time at which the comparator (COMP) outputs the comparative signal.