It is an object to provide a laser apparatus, a laser irradiating method
and a manufacturing method of a semiconductor device that make laser
energy more stable. To attain the object, a part of laser beam emitted
from an oscillator is sampled to generate an electric signal that
contains as data energy fluctuation of a laser beam. The electric signal
is subjected to signal processing to calculate the frequency, amplitude,
and phase of the energy fluctuation of the laser beam. The transmittance
of a light amount adjusting means is controlled in order that the
transmittance changes in antiphase to the phase of the energy fluctuation
of the laser beam and with an amplitude capable of reducing the amplitude
of laser beam emitted from the oscillator, the control being made based
on the phase difference between the phase of a signal that is in
synchronization with oscillation of laser beam emitted from the
oscillator and the phase calculated, on the energy ratio of the sampled
laser beam to laser beam emitted from the oscillator, and on the
frequency and amplitude calculated. In the light amount adjusting means,
energy of the laser beam oscillated from the oscillator energy is
adjusted.