To provide a lubricant deterioration detecting device having a high degree of freedom in arranging within the bearing assembly or the like and capable of stably detecting the status of deterioration of the lubricant without being affected by the thickness of the lubricant and a change in temperature and also to provide a detector incorporated bearing assembly equipped with such lubricant deterioration detecting device, the lubricant deterioration detecting device includes a light source, a plurality of light detecting components for detecting rays of light emitted from the light source and subsequently transmitted through a lubricant, and a determining unit. The plural light detecting components have respective light detecting faces sequentially displaced in position. The determining unit compares respective signal strengths of outputs from the plural light detecting components to detect the status of deterioration of the lubricant.