PROBLEM TO BE SOLVED: To make a detection performance of magnetic detection means certain and the management of the construction.;SOLUTION: Not only a suction hole material 3 is arranged to a fixing hole 2 provided to the side wall 1a of the bathtub 1 but also the hole flange of the fixing hole 2 is held water-tightly and the suction hole material 3 is fixed to the fixing hole 2. At a position that is outer than the peripheral from the connection hole 4 which is led out of the bathtub 1 of the suction hole material 3 but inner than the inside perimeter from the hole flange of the fixing hole 2, a partition wall 10 which partitions inside of the bathtub 1 and outside of the bathtub 1 by a roughly parallel face with the side wall 1a of the bathtub 1 is almost provided around the entire perimeter of the suction hole material 3. To the outer face of the bathtub 1 of the partition wall 10, a magnetic substance 11 and a magnetic detection means A are installed. A suction cover 7 is covered to cover the suction hole 12 of the suction hole material 3, and at the same time, a magnet 13 which is integrally provided to the suction cover 7 is arranged inside of the bathtub 1 and magnetizes a magnetic substance 11 through the partition wall 10.;COPYRIGHT: (C)2003,JPO
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