Processing state determination method and determination device

The method for determining the machining state includes: a step for detecting, using an optical sensor, at least one component among heat radiation, visible light, and reflected light generated in a welded portion formed on the surface of a workpiece by irradiating the workpiece with laser light; a step for acquiring a signal indicating a change in at least one component in a time interval from the start of welding of the workpiece to the end of welding; a step for calculating a feature amount on the basis of the signal strength of the signal in a given interval among the time intervals; a step for inputting the calculated feature amount into a determination model for determining the processing state, and determining the amount of a gap occurring between the overlapping surfaces of the workpiece in the laser irradiation direction as the processing state; and a step for outputting the determined gap amount as a determination result. The determination model is constructed on the basis of training data including