A magnetoresistive device includes a magnetoresistive film made of permalloy alloy. This thin film is formed on a substrate by sputtering or vapor deposition method. Thereafter, the thin film is heated to a temperature between 200.degree. C. and 350.degree. C. by flowing an electric current therethrough or irradiating the thin film with an electron or laser beam. It is desirable that the heat treatment is effected in an alternating magnetic field. The permalloy alloy may contain at least one element including Rh, Ru, Mo, Cr and V.