200 ml minute a sodium carbonate solution 1 0 mol l at a flow rate of 60 ml minute and a sodium hydroxide aqueous solution na 3 0 mol l at a flow rate of about 160 ml minute were supplied to a reaction vessel having a volume of 3 l and having overflow equipment in which 1 l of water had been placed in advance with a metering pump with stirring to carry out a coprecipitation reaction while the ph of the mixture was kept at about 9 0 9 3 by adjusting the flow rate of the sodium hydroxide aqueous solution from 20 minutes after overflow the reaction mixture was recovered for 30 minutes and it was filtered under reduced pressure then washed with 6 liters of a sodium carbonate aqueous solution 0 2 mol l and washed with water the water washed product was added to water and the resultant mixture was subjected to dispersion treatment with an agitator then part of the resultant mixture was placed in an autoclave and hydrothermally treated at 140 u00b0 c for 15 hours the hydrothermally treated product was taken out filtered under reduced pressure washed with water dried at about 120 u00b0 c for 10 hours and pulverized thereby producing an intended agent as a powder.