A method for adjusting a light emitting device comprising: a laser light source that generates a relaxation oscillation immediately after an electric current is applied thereto for driving light emission; a capacitive reactance circuit that exhibits low impedance immediately after the electric current is applied thereto and which is chargeable with electrical charge; a resistance circuit that discharges electrical charge charged in the capacitive reactance circuit after a predetermined time passes after the electric current is applied; a differentiation circuit formed of the capacitive reactance circuit and the resistance circuit, which are connected in parallel; and a switching element, wherein the laser light source and the differentiation circuit are connected in series, and the switching element is configured to switch on or off for application of voltage to the laser light source and the differentiation circuit, the method comprising adjusting characteristics of the capacitive reactance circuit and the resistance circuit so that one of the oscillations of the relaxation oscillation is obtained, characterized in that at least one of capacitance value and impedance of the capacitive reactance circuit is adjusted so that electric current flows in the laser light source at not less than a threshold current value for oscillation immediately after the electric current is supplied to the capacitive reactance circuit and so that the electric current flows in the laser light source at an amount at which the light emission of the laser light source does not come to be an oscillation stationary state, wherein the characteristics of the capacitive reactance circuit and the resistance circuit are adjusted so that light of only initial oscillation of the relaxation oscillation is emitted. a laser light source that generates a relaxation oscillation immediately after an electric current is applied thereto for driving light emission; a capacitive reactance circuit that exhibits low impedance immediately after the electric current is applied thereto and which is chargeable with electrical charge; a resistance circuit that discharges electrical charge charged in the capacitive reactance circuit after a predetermined time passes after the electric current is applied; a differentiation circuit formed of the capacitive reactance circuit and the resistance circuit, which are connected in parallel; and a switching element, wherein the laser light source and the differentiation circuit are connected in series, and the switching element is configured to switch on or off for application of voltage to the laser light source and the differentiation circuit, the method comprising adjusting characteristics of the capacitive reactance circuit and the resistance circuit so that one of the oscillations of the relaxation oscillation is obtained, characterized in that at least one of capacitance value and impedance of the capacitive reactance circuit is adjusted so that electric current flows in the laser light source at not less than a threshold current value for oscillation immediately after the electric current is supplied to the capacitive reactance circuit and so that the electric current flows in the laser light source at an amount at which the light emission of the laser light source does not come to be an oscillation stationary state, wherein the characteristics of the capacitive reactance circuit and the resistance circuit are adjusted so that light of only initial oscillation of the relaxation oscillation is emitted.