LUBRICANT DETERIORATION SENSOR, SPEED REDUCER FOR INDUSTRIAL ROBOT AND INDUSTRIAL ROBOT

PROBLEM TO BE SOLVED: To provide a lubricant deterioration sensor capable of instantaneously identifying a type and an amount of a contaminant in lubricant of a machine.SOLUTION: A lubricant deterioration sensor 139b which is installed in an industrial robot and for detecting deterioration of lubricant 131a of the industrial robot includes: a white LED 72 which emits light; an RGB sensor 73 which detects the color of the received light; a gap formation member 60 in which a gap 60a for oil for the lubricant 131a to penetrate is formed; a support member 30 which supports the white LED 72, the RGB sensor 73 and the gap formation member 60; and a housing 20 for being fixed to the industrial robot. The gap formation member 60 transmits the light to be emitted by the white LED 72, the gap 60a for oil is arranged on an optical path from the white LED 72 to the RGB sensor 73, and the housing 20 rotatably supports the support member 30 so that the direction of an opening of the gap 60a for oil changes when the support member 30 rotates.