Provided is an abnormality detection device with which an abnormality of a machining state of a machine tool can be detected based on information on a section where machining is actually performed in machine tool-based machining. The abnormality detection device detects an abnormality of a machining state of a machine tool machining a workpiece with a tool. The machine tool includes a determination unit determining the machining state by using information related to an actual cutting section in the tool-based machining of the workpiece in the machine tool. The determination unit performs the machining state determination by using a deviation in position and length of a section recognized as the actual cutting section and a physical quantity in the actual cutting section acquired from the machine tool.