To make quickly and surly diagnosable even a wear state based on the repetition of progressing and stopping in the early stages to middle stages.

The foreign matter diagnostic device 1 is composed of a cyclotron 5 sedimenting a foreign matter 4 mixed in lubricating oil 2 in a passage system 3 of the lubricating oil 2, a transparent plate 6 storing a sedimenting foreign matter 4 thereon, and a metallograph 7 mounted on the transparent plate 6 and detecting a foreign matter property. The transparent plate 6 stores the sedimenting foreign matter 4, and the property of the stored foreign matter 4 is easily detected by the metallograph 7. Thereby, the nature of the lubricating oil 2 flowing the passage system 3 is observed in a time series and the occurred foreign matter 4 in predetermined steps formed by dividing the time series are observed so as to grasp the nature of the lubricating oil 2 quickly and surly.