A magneto-sensitive wire for a magnetic sensor with both measurement range expansion and environment resistance performance improvement, includes a Co-based alloy containing more Fe than a reference composition that is amorphous overall and exhibits zero magnetostriction. The Co-based alloy may have an Fe ratio (Fe/(Co+Fe+Ni)) of 6.1% to 9.5%. The Fe ratio is an atomic fraction of the Fe amount with respect to the total amount of a magnetic element group consisting of Co, Fe, and Ni. By heating an amorphous wire of a Co-based alloy at a temperate at least equal to a crystallization start temperature and lower than a crystallization end temperature, allows the magneto-sensitive wire to have a composite structure in which crystal grains are dispersed in the amorphous phase. The magneto-sensitive wire's anisotropy field is, for example, 5 to 70 Oe and the stress sensitivity, indicative of magnetostriction, is −30 to 30 mOe/MPa.