To demonstrate a magnetic effect effectively when immersed in water, to prevent the deterioration of water quality, and to promote the growth of plants by providing a backing which has a plate-shaped main body and is magnetized permanently in a plane and forming through holes which penetrate the backing vertically.

Through-holes 12 are formed in a magnet 10 which is magnetized in parallel with the opposite side from a backing which is molded by mixing polyamide with magnetic powder into the shape of a rectangular plate and N poles and S poles are arranged alternately at intervals of several mm. By installing the through-holes 12 in this way, the magnetic effect of the magnet 10 acts effectively on water, and especially prevents the deterioration of the quality of stored water appropriately to maintain the growth environment of plants and animals appropriately. The shape of the magnet 10 is not necessarily rectangular and can be circular and of other shapes. Moreover, the magnet 10 is not limited to a plastic magnet, and the raw material is not restricted in particular so long as it is a permanent magnet.