A laser beam irradiation device according to the present invention is so constructed that it may increase or decrease the excitation electric current flowing through its semiconductor laser diode to enable a user to selectively practice both non-thermal and photo-thermal laser treatments.CPU 41 is responsive to switching-on and -off of an output-power switching unit S2 for sending a reference signal to an amplifier 472 via an I/O port 44 and a D/A converter 471, so that the amplifier 472 may provide at its output terminals a reference voltage, with respect to which a control reference voltage for a desired laser output can be determined.Semiconductor laser diode 35 radiates the laser beam from its opposite sides, and a photo-diode 473 receives one of the opposite laser beams to provide a light-accepting voltage at its output terminals. The so acquired light-accepting voltage is directed to an amplifier 474 for amplification, thereby obtaining the monitor voltage.The reference voltage and the monitor voltage are applied to a differential amplifier 475, and a differential voltage is amplified by an amplifier 476. Thus a controlled working electric current is made to flow in semiconductor laser diode 35 so that it may be energized and oscillated at the set output level.