To provide an emission optical system for a laser beam machine capable of easily changing the beam cone angle.

A fixed aperture 13 and a movable aperture 15 to make the laser beam pass from an optical fiber are provided on a machining head provided with a collimate lens 17 to collimate the laser beam from the optical fiber 11 and a work lens 18 to converge the collimated laser beam. The movable aperture is supported by a cooling block 16 movably along the optical axis, and when it is moved in the direction away from the optical fiber, a part of the laser beam is reflected, and the beam diameter of the laser beam incident on the collimate lens 17 is changed. As a result, the beam cone angle of the laser beam incident on a work is changed.