PURPOSE: To detect rubbing with high sensitivity by providing an acoustic sensor in a bearing part of a rotary machine to detect high-frequency abnormal sounds which are generated when rubbing occurs.

CONSTITUTION: Acoustic sensors 5a and 5b are provided on sliding bearings 2a and 2b of a rotor 1. High-frequency abnormal sounds due to rubbing which occurs in the position marked with a black star are propagated in the rotor 1 and are detected by acoustic sensors 5a and 5b. Their outputs pass amplifiers 6a and 6b, filters 7a and 7b, and detecting circuits 8a and 8b, and signals f(t) and g(t) are inputted to a cross-correlation function analyzer 9, and a cross-correlation function is calculated and is sent to a monitor 10. Thus, presence/absence of rubbing is detected with a high sensitivity.