To provide a coherence control method for a broad-stripe semiconductor laser device, which improves the high-power semiconductor laser device beam quality and oscillation spectrum.

A laser beam emitted from an active layer 2 of a semiconductor laser device 1 is collimated by a collimator lens 3, reflected by a reflector 4, and then outputted upward. Part of the laser beam out of the collimator lens 3, however, is fed back to the active layer 2 of the semiconductor laser device 1 via a spatial filter constituted of a slit 5 and other elements, arranged in the far view field (Fourier plane) which is apart from the collimator lens 3 by approximately a focal length, and of a diffraction grating mirror 6. The oscillation spectrum is narrowed and the laser beam is stabilized and remarkably improved in the laser beam quality.