Provided is a working machine capable of accurately detecting a contaminant in grease contained in a slewing mechanism. The working machine includes a lower travelling body, an upper slewing body, and the slewing mechanism. The slewing mechanism includes: a ring gear fixed to the lower travelling body and having inner teeth; a pinion disposed on the upper slewing body and rotated in mesh with the inner teeth of the ring gear; and a grease bath disposed along an inner circumference of the ring gear for storing grease. The upper slewing body includes a sensor for detecting a contaminant having a potential for contaminating and deteriorating the grease, and a sensor supporting member supporting the sensor at a position that allows the sensor to revolve while being immersed in the grease in the grease bath.