PURPOSE: To prevent reduction in the performance and minimize wear of bladder by providing a bearing of a float capable of being axially slidable to a rod projected into the bladder, and providing rollers at a part in which the float abuts on the bladder.

CONSTITUTION: A non-magnetic hollow rod 5 projected into a bladder 2 is provided in the central part of a cap 4, a float 8 is fitted slaidably into the rod 5 through balls of a bearing 14, and a cover 15 is provided outside the float. Further, roller 16 are arranged to a part 8a where the float abuts on the bladder, the rollers 16 are provided so as to be projected outside the cover 15, and a switch 13 which operates as a magnet sensor 10 of the float 8 approaches it, is provided in the rod 5. The float can thus be smoothly slided without reducing the performance, and as a result, the amount of wear of the bladder can be reduced to prevent damage to the bladder and to obtain a longer life.