extremely simple configuration is featured by the photoelectric loop connection for effecting the feedback control such as respective variations of the photocurrent of the photodetector the exciting current of the laser diode the output light and again the aforesaid photocurrent are performed successively in circulation under the basic configuration as shown in fig1 however this basic configuration of the device can be variously modified in addition thereto the semiconductor laser diode ld consisting in this basic configuration can be substituted by various kinds of laser emitting elements such as gas solid and liquid for instance dye lasers whereas various kinds of photodiodes phototransistors avalanche photodiodes and the like can be employed for the photodetector pd one of the most simple modifications of the device is shown in fig4 a in this modification a resistor r b which is desirably made adjustable is used mainly for setting up the bias current of the laser diode ld and a resistor r g is used for branching and setting up the feedback controlling current of the laser diode ld and further a resistor r s is used for protecting the laser diode ld and as a result affects the response time of the device the source voltage v b in this modification is supplied by a stabilized power supply sps for stabilizing the operation.