To provide a leaking liquid sensor capable of rapidly and accurately detecting leaking liquid.

This leaking liquid sensor includes a translucent part made of transparent or semi-transparent material, radiates survey light to the translucent part with the translucent part faced to the inside of a leaking liquid detecting region, and detects the leaking liquid based on change of quantity of light of reflected light reflected on the boundary between the translucent part and the detecting region. The leaking liquid sensor 18 has a mounting means 34 for setting the leaking liquid sensor 18 into the detecting region 22. The mounting means 34 comprises a grounding part 36 connecting to the detecting region 22, and mounting parts 66, 68, and 70 stood on the grounding part 36 that can mount the leaking liquid sensor 18 while the grounding part 36 is parallel with the translucent part 20 or the grounding part and the translucent part form a predetermined clearance.