The semiconductor light source drive device according to the present disclosure includes: a switching power source that is connected in series with a semiconductor light source element and a constant current circuit to output a predetermined voltage to the semiconductor light source element and the constant current circuit; a detection circuit that detects a voltage at a connecting point between the semiconductor light source element and the constant current circuit; and a controller that controls the voltage output from the switching power source based on the voltage detected by the detection circuit, and controls a current value of the constant current circuit based on a switching signal. The controller, prior to varying the current value of the constant current circuit, varies the voltage output from the switching power source in accordance with an amount to be varied in the current value of the constant current circuit.