A laser processing device comprising: a seed light source emitting seed light; an excitation light source (3, 9A, 9B) emitting excitation light; and a light-amplifying fiber (1, 8) arranged such that the seed light is amplified by coupling the seed light and the excitation light into it; wherein the seed light source is configured to repeatedly generate, as the seed light, pulse trains including a plurality of light pulses, the seed light source generating the pulse trains with a predetermined period; a time interval (tp) between the plurality of light pulses is shorter than an interval (tprd) between the pulse trains; and a number, a pulse width, an amplitude and an interval of the light pulses are controllable independently from each other, wherein the amplitude of the light pulse is changed according to a processing condition so that an envelope of a waveform of each pulse train increases strictly monotonically with respect to time, wherein the seed light source comprises a semiconductor laser; and the laser processing device further comprises a drive circuit driving the semiconductor laser using changeable waveform data defining the waveform of the pulse train, the drive circuit comprising: a storage section for storing the waveform data; a digital signal generator for reading out the waveform data from the storage unit and outputting, based on the waveform data, a clock signal (DAC_clk) and a data signal (DAC_data) containing digital data; and a digital/analog converter configured to receive the clock signal and the data signal from the digital signal generator and configured to convert the data indicated by the data signal into an analog signal (Idac) in synchronization with the clock signal, wherein the analog signal (Idac) controls a drive current that is supplied to the seed light source, and the drive current controls intensity of the emitted seed light. a seed light source emitting seed light; an excitation light source (3, 9A, 9B) emitting excitation light; and a light-amplifying fiber (1, 8) arranged such that the seed light is amplified by coupling the seed light and the excitation light into it; wherein the seed light source is configured to repeatedly generate, as the seed light, pulse trains including a plurality of light pulses, the seed light source generating the pulse trains with a predetermined period; a time interval (tp) between the plurality of light pulses is shorter than an interval (tprd) between the pulse trains; and a number, a pulse width, an amplitude and an interval of the light pulses are controllable independently from each other, wherein the amplitude of the light pulse is changed according to a processing condition so that an envelope of a waveform of each pulse train increases strictly monotonically with respect to time, wherein the seed light source comprises a semiconductor laser; and the laser processing device further comprises a drive circuit driving the semiconductor laser using changeable waveform data defining the waveform of the pulse train, the drive circuit comprising: a storage section for storing the waveform data; a digital signal generator for reading out the waveform data from the storage unit and outputting, based on the waveform data, a clock signal (DAC_clk) and a data signal (DAC_data) containing digital data; and a digital/analog converter configured to receive the clock signal and the data signal from the digital signal generator and configured to convert the data indicated by the data signal into an analog signal (Idac) in synchronization with the clock signal, wherein the analog signal (Idac) controls a drive current that is supplied to the seed light source, and the drive current controls intensity of the emitted seed light. a storage section for storing the waveform data; a digital signal generator for reading out the waveform data from the storage unit and outputting, based on the waveform data, a clock signal (DAC_clk) and a data signal (DAC_data) containing digital data; and a digital/analog converter configured to receive the clock signal and the data signal from the digital signal generator and configured to convert the data indicated by the data signal into an analog signal (Idac) in synchronization with the clock signal, wherein the analog signal (Idac) controls a drive current that is supplied to the seed light source, and the drive current controls intensity of the emitted seed light.