To enhance durability of a rubber ball in the case where water hammer is prevented from being generated using the rubber ball or the like.

Rubber balls 11 are stored and held inside a main body 2 to be freely inserted into a screen along the axial direction of the screen of a strainer. Opening parts X are formed in both end parts of the main body 2, and opening parts Y are formed by frame materials 5 and reinforcing rings 6 in a circumferential face of the main body 2. Locking members 7, 8 for abutting elastically to the screen to lock it are fixed to end rings 3, 4 in both end parts of the main body 2 radially in the radial direction.