To provide an optical system and a laser processing device capable of setting easily energy strength distribution of laser beam on a processed point, so that the energy strength distribution is higher on a peripheral area than on a center area.

An optical system 7 comprises: a collimator lens group 8 emitting laser beam having a parallel flux; an axicon lens 9 which is conical shape and to which the emitted beam from the collimator lens group 8 injects; and a focus lens group 10 condenses the emitted beam from the axicon lens 9. A conical face of the axicon lens 9 locates on the collimator lens group 8 side and a plane face of the axicon lens 9 locates on the focus lens group 10 side.