A laser welding method is capable of easily restraining poor welding when spatters adhere to a protective glass of an optical system. The laser welding method includes a step of calculating a decrease-amount of the laser power before laser welding is performed by irradiating a welding portion of a workpiece with the laser beam having a predetermined power. The step of calculating the decrease-amount includes irradiating the welding portion with an inspecting laser beam having a power smaller than the predetermined power, receiving a return beam of the inspecting laser beam, measuring an intensity of the return beam, and comparing the intensity of the return beam with a standard intensity to calculate an amount of decrease in power of the inspecting laser beam at the welding portion.