A device for estimating machining dimensions of a machine tool which employs tool members each being rotatably driven by a driving unit includes: a vibration sensor (2); a characteristics extracting unit (3) for extracting amounts of characteristics from an output of the vibration sensor; a neural network (1) for classifying the amounts of characteristics into categories; and a conversion unit (6). Amounts of characteristics of generated output by racing the tool member are used for training the neural network, and inputted again to the trained competitive learning neural network to excite neurons so that the relationships between Euclidean distances and machining dimensions of workpieces are registered in the conversion unit. The Euclidean distances are obtained between weight vectors of the excited neurons and respective corresponding training samples, and the machining dimensions are obtained when the workpieces are machined by the tool members at the same condition as the respective corresponding training samples are obtained.