To easily deal with a change in a treated water volume by enhancing the treatment effect of water with extremely simple constitution and easily assembling magnets, to prevent the circulation of crushed 'BAKUHANSEKI' (siliceous stone produced in Japan) and to prevent the clogging of the granite porphyry between ceramic balls.

When a filter 16 is arranged on the upstream side of a magnet body 13 and if the 'BAKUHASEKI' 15 is bombarded and crushed by the flow of the water, the 'BAKUHANSEKI' pieces enter the inside of a water storage tank 11. Since the 'BAKUHANSEKI' pieces are captured by the filter 16 through a pump 12 by the suction of the pump 12, the clogging of the 'BAKUHANSEKI' pieces between the ceramic balls 14 is surely prevented and the good state of the circulating flow of the water is maintained. When the crushed 'BAKUHANSEKI' pieces are captured by the filter 16, minerals are eluted from the 'BAKUHANSEKI' pieces as well by the circulation of the water and, therefore, the water treatment effect is additionally enhanced.