Abstract: In a leak sensor including a light-transmitting part 16 constituted by a transparent or semitransparent material, the leak sensor irradiating the light-transmitting part 16 with probe light L in a state where the light-transmitting part 16 is provided so as to face into a region subjected to leak detection 26, and detecting a leaked liquid according to a change in quantity of light L* reflected by a boundary between the light-transmitting part 16 and the region subjected to leak detection 26; the leak sensor 10 comprises an attachment 28 for installing the leak sensor 18 in the region subjected to leak detection 26; the attachment 28 comprising a ground portion 30 grounded on the region subjected to leak detection 26; and mounting portions 36, 38, 40, rising from the ground portion 30, adapted to mount the leak sensor 18 in a state where the light-transmitting part 16 is parallel to the ground portion 30 and in a state where a predetermined space is formed therebetween.