To discriminate machining of a work without making an operation troublesome or flawing the work.

A color identifying sensor 30 sequentially projects lights (red, green, blue) with a plurality of wavelengths orthogonally to a tooth flank 50a of a gear 50 in order, and detects a received amount of each light regularly reflected from the tooth flank 50a. A measuring unit 40 receives a signal related to the received amount of each light from the color identifying sensor 30, determines whether or not the tooth flank 50a has been machined based on a ratio of the received amounts of respective lights, and thus discriminates whether or not the gear 50 has been machined.