To provide the beam mode converting optical system which can actualize a beam mode having its radial output distribution varied.

The laser light from a collimator lens 12 is split by a 1st partial reflecting mirror 13 by being partially transmitted and partially reflected. The laser light traveling along an optical path 1 passes through the opening part of a 2nd partial reflecting mirror 18 and enters an optical fiber 19 through a focusing lens 20. The laser light traveling along an optical path 2 passes through a 1st total reflecting mirror 14 and made incident to a 1st prism 15. The 1st prism 15 refracts the incident laser light so that it travels while diverged in the circumferential direction and a 2nd prism 16 converts the laser light into parallel light. This laser light is reflected by a 2nd total reflecting mirror 17 and the 2nd partial reflecting mirror 18 and put together with the laser light of the optical path 1.