The present invention relates to a laser oscillator, especially to a laser oscillator suitable for ultrafine manufacturing such as an apparatus for manufacturing of a semiconductor wherein the laser oscillator is provided with constant directivity chronically, although the directivity is not so fine, for avoiding a blurred image spatially and chronically in order to obtain a stable mask in the ultrafine manufacturing. The laser oscillator includes apparatus for enabling at least one of selection of the directivity of parallel light from a resonator between a whole reflector and a laser medium; an optical system for stopping oscillation of the laser medium before a laser beam, which is generated at the resonator, becomes a single mode and for eliminating the laser beam having poor directivity; prevention of the laser medium from re-entry of unnecessary natural emitting light which is generated in the laser medium; and providing a value of a diameter of an aperture of the optical system for maintaining directivity so as to make an order number of the transverse mode high.