It is an object to provide a laser apparatus, a laser irradiating method and a manufacturing method of a semiconductor device that can perform uniform a process with a laser beam to an object uniformly. The present invention provides a laser apparatus comprising an optical system for sampling a part of a laser beam emitted from an oscillator, a sensor for generating an electric signal including fluctuation in energy of the laser beam as a data from the part of the laser beam, a means for performing signal processing to the electrical signal to grasp a state of the fluctuation in energy of the laser beam, and controlling a relative speed of an beam spot of the laser beam to an object in order to change in phase with the fluctuation in energy of the laser beam.