A laser welding device for welding a weld part with a laser light, comprising: an irradiator (20, 20A) overlapping the laser light and a measurement light coaxially with each other and applying the laser light and the measurement light to the weld part, the measurement light having a wavelength different from a wavelength of the laser light; a measuring instrument repeatedly measuring a weld penetration depth of the weld part based on the measurement light that is emitted from the irradiator (20, 20A) and is reflected on the weld part, thereby generating a plurality of measured values; and a determiner configured to determine the weld penetration depth of the weld part based on one or more measured values, where among the plurality of measured values, or on an average value of the one or more measured values,- characterized in that the determiner is configured to sample the one or more measured values included within a predetermined range from the deepest side measured value of the plurality of measured values, and further comprising an irradiation position changer (26, 27) which is configured to change an irradiation position of the measurement light such that the irradiation position moves on a predetermined welding path, and change an optical axis position as the irradiation position of the measurement light such that the optical axis position moves within a region having a radius of rotation (r) that is smaller than 1/2 of a spot diameter of the laser light, and change the irradiation position of the measurement light so that the irradiation position orbitally moves around a center of rotation that moves on the predetermined welding path, wherein the measuring instrument is configured to generate the plurality of measured values of the weld penetration depth while the measurement light is being moved. an irradiator (20, 20A) overlapping the laser light and a measurement light coaxially with each other and applying the laser light and the measurement light to the weld part, the measurement light having a wavelength different from a wavelength of the laser light; a measuring instrument repeatedly measuring a weld penetration depth of the weld part based on the measurement light that is emitted from the irradiator (20, 20A) and is reflected on the weld part, thereby generating a plurality of measured values; and a determiner configured to determine the weld penetration depth of the weld part based on one or more measured values, where among the plurality of measured values, or on an average value of the one or more measured values,- characterized in that the determiner is configured to sample the one or more measured values included within a predetermined range from the deepest side measured value of the plurality of measured values, and further comprising an irradiation position changer (26, 27) which is configured to change an irradiation position of the measurement light such that the irradiation position moves on a predetermined welding path, and change an optical axis position as the irradiation position of the measurement light such that the optical axis position moves within a region having a radius of rotation (r) that is smaller than 1/2 of a spot diameter of the laser light, and change the irradiation position of the measurement light so that the irradiation position orbitally moves around a center of rotation that moves on the predetermined welding path, wherein the measuring instrument is configured to generate the plurality of measured values of the weld penetration depth while the measurement light is being moved.