PURPOSE: To improve the strength, corrosion resistance and stress corrosion cracking resistance of a magnetic alloy used as a filter medium for magnetic separation by blending a specified amt. of Fe, Ni and two or more kinds of metalloid elements selected from Si, B, P and C.

CONSTITUTION: To an alloy of a compsn. of formula (Fe1-aNia)100-yXy (where X is two or more out of Si, B, P and C; 0.3≤a≤0.65 and 15≤y≤30) is optionally added 0.2W4.5at% of one or more out of Mo, W, Cr, Nb, Ti and Al. This alloy is melted and solidified by quenching to form an amorphous state. The amorphous alloy is then worked into desired form to manufacture a filter element for magnetic separation. This amorphous alloy has high corrosion resistance and esp. superior stress corrosion cracking resistance, so it is suitable for use as the filter medium of a magnetic separator for treating waste water, etc.