The present invention relates to a passively Q-switched laser with a dual-cavity configuration to obtain a symmetrical laser pulse with a short and variable pulse width. The passively Q-switched laser comprises: a laser medium and a passively Q-switching medium; a cavity mirror capable of fully reflecting the respective wavelengths emitted from the laser medium and the passively Q-switching medium; a main cavity mirror composed of a dichroic mirror having non-reflective characteristics for the light emitted from the passively Q-switching medium and reflective characteristics for the light emitted from the laser medium with a reflectivity of 10% or more; and, an auxiliary cavity mirror composed of a dichroic mirror having non-reflective characteristics for the light emitted from the laser medium and reflective characteristics for the light emitted from the passively Q-switching medium with a reflectivity of 10% or more and capable of moving transversely against the laser beam.