A method of crystallizing a semiconductor film including splitting a pulse laser beam oscillated from a laser oscillator, and synthesizing the split pulse laser beams after the split pulse laser beams have propagated through optical paths different in optical path length, modulating the synthesized pulse laser beam into a pulse laser beam by a phase modulating element, and irradiating a non-single-crystal film formed on a substrate with the laser beam to crystallize the non-single-crystal film. Splitting the pulse laser beam and synthesizing the split pulse laser beams are performed using at least three optical splitting/synthesizing units arranged in order, and include sequentially splitting one pulse laser beam split by one optical splitting/synthesizing unit by succeeding splitting/synthesizing unit, and synthesizing the other pulse laser beam split by one optical splitting/synthesizing unit with the other pulse laser beam split by preceding splitting/synthesizing unit.