In the case where, e.g., because of provision of a high-gain and
high-energy Q-switched laser oscillator in a laser system in a MOPA
configuration, the duration .tau.z from the time point when
oscillation-stage Q switches (13a and 13b) start the gate ON to the time
point when a pulse laser beam (18) starts to grow is shorter than the
fall time .tau.f of amplification-stage Q switches (13c, 13d, and 13e),
by implementing control in such a way that the gate-ON timing of the
oscillation-stage Q switches (13a and 13b) lag behind the gate-ON timing
of the amplification-stage Q switches (13c, 13d, and 13e) by a
predetermined time, the loss in the pulse laser beam (18) at the
amplification-stage Q switches (13c, 13d, and 13e) can be prevented,
while the gain deterioration due to a spontaneously amplified ray (17)
produced in the amplification stage is being suppressed. Therefore, a
high-energy pulse laser beam can efficiently be obtained.