A high power pulsed laser light generation device comprising: - a master clock generator that generates a master clock signal; - an optical oscillator that generates a pulsed light synchronized with the master clock signal; - an optical amplifier that amplifies the pulsed light emitted from the optical oscillator to output a high power pulsed light; - a pump semiconductor laser that generates a pulsed light for pumping the optical amplifier; - a driving unit that drives the pump semiconductor laser by a pulsed driving current synchronized with the master clock signal; and - a control unit which controls the driving unit and controls a gain of the optical amplifier for each pulse by changing a pulse width of the pulsed drive current from driving unit so as to change the pulse width (tm) of the pumping pulsed light, wherein - the control unit is configured such that immediately after changing the state of the high power pulsed laser light generation device from an OFF state where the driving of the pump semiconductor laser stops and the high power pulsed light is not output from the optical amplifier to an ON state where the driving of the pump semiconductor laser starts and the high power pulsed light is output from the optical amplifier, the pulse width (tm) of the pulsed light for pumping is adjusted in order for the gain of the optical amplifier immediately before the emission of an output pulsed light to approach a maximum gain in the stable ON state of the optical amplifier, and the pulse width (tm) of the pulsed light for pumping corresponding to a first pulse of the output pulsed light from the optical amplifier is longer than the pulse width (tm) of the pulsed light for pumping in the stable ON state of the optical amplifier. - a master clock generator that generates a master clock signal; - an optical oscillator that generates a pulsed light synchronized with the master clock signal; - an optical amplifier that amplifies the pulsed light emitted from the optical oscillator to output a high power pulsed light; - a pump semiconductor laser that generates a pulsed light for pumping the optical amplifier; - a driving unit that drives the pump semiconductor laser by a pulsed driving current synchronized with the master clock signal; and - a control unit which controls the driving unit and controls a gain of the optical amplifier for each pulse by changing a pulse width of the pulsed drive current from driving unit so as to change the pulse width (tm) of the pumping pulsed light, wherein - the control unit is configured such that immediately after changing the state of the high power pulsed laser light generation device from an OFF state where the driving of the pump semiconductor laser stops and the high power pulsed light is not output from the optical amplifier to an ON state where the driving of the pump semiconductor laser starts and the high power pulsed light is output from the optical amplifier, the pulse width (tm) of the pulsed light for pumping is adjusted in order for the gain of the optical amplifier immediately before the emission of an output pulsed light to approach a maximum gain in the stable ON state of the optical amplifier, and the pulse width (tm) of the pulsed light for pumping corresponding to a first pulse of the output pulsed light from the optical amplifier is longer than the pulse width (tm) of the pulsed light for pumping in the stable ON state of the optical amplifier.