PURPOSE:To reduce the size of whole water immersion sensor tool by providing a slider whose one end can oscillate about the other end and operating this slider by the water-absorption expansion of a water-absorbing expansion material. CONSTITUTION:The water immersion sensor tool is so constituted that when water entering a closure enters a casing 40, the water-absorbing expansion material 46 expands, and consequently the slider 45 rotates around the axis 43 of one end of the slider 45. Consequently, a coated optical fiber A is pressed to abut on the slider 45 and a stopper part 46 deforms, so the transmission loss of the optical fiber deforms and water immersion into the closure is detected. Thus, the slider 45 rotates around the axis 45 of one end, so the other end surface rotates with a large force through lever operation even if the water- absorption expansion quantity of the water absorbing expansion material 46 is slight. Thus, the whole water immersion sensor tool is reduced in size and variation in the transmission loss of the optical fiber is obtained in case of water immersion.
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