A laser device for a laser beam machine for cutting or welding, comprising: at least one light source; a delivery fiber that is configured to propagate laser light emitted from the light source; and a first light detection unit and a second light detection unit configured to detect a part of light propagating in a direction opposite to a propagation direction of the laser light through the delivery fiber, wherein the first light detection unit detects first light included in a wavelength band of visible light, and the second light detection unit detects second light included in a wavelength band of near-infrared light, wherein the laser device further comprises: a monitor fiber that is optically coupled to the delivery fiber and is configured to propagate part of the light propagating in the direction opposite to the propagation direction of the laser light in the delivery fiber, wherein one end portion of the monitor fiber is connected to the first light detection unit, characterized in that the second light detection unit is a Rayleigh monitor disposed at a position facing an outer peripheral surface of the monitor fiber for detecting Rayleigh light of the light propagating through the monitor fiber and in that the first light detection unit is configured so that the erroneous detection of a fiber fuse is suppressed and the fiber fuse is accurately detected. at least one light source; a delivery fiber that is configured to propagate laser light emitted from the light source; and a first light detection unit and a second light detection unit configured to detect a part of light propagating in a direction opposite to a propagation direction of the laser light through the delivery fiber, wherein the first light detection unit detects first light included in a wavelength band of visible light, and the second light detection unit detects second light included in a wavelength band of near-infrared light, wherein the laser device further comprises: a monitor fiber that is optically coupled to the delivery fiber and is configured to propagate part of the light propagating in the direction opposite to the propagation direction of the laser light in the delivery fiber, wherein one end portion of the monitor fiber is connected to the first light detection unit, characterized in that the second light detection unit is a Rayleigh monitor disposed at a position facing an outer peripheral surface of the monitor fiber for detecting Rayleigh light of the light propagating through the monitor fiber and in that the first light detection unit is configured so that the erroneous detection of a fiber fuse is suppressed and the fiber fuse is accurately detected. a monitor fiber that is optically coupled to the delivery fiber and is configured to propagate part of the light propagating in the direction opposite to the propagation direction of the laser light in the delivery fiber, wherein one end portion of the monitor fiber is connected to the first light detection unit, characterized in that the second light detection unit is a Rayleigh monitor disposed at a position facing an outer peripheral surface of the monitor fiber for detecting Rayleigh light of the light propagating through the monitor fiber and in that the first light detection unit is configured so that the erroneous detection of a fiber fuse is suppressed and the fiber fuse is accurately detected.