A magnetoresistive sensor is fabricated as follows. First of all, first antiferromagnetic layers are created on the upper surfaces on both sides of a lower-gap layer, sandwiching a track width on the upper surface of the lower-gap layer. Then, a free magnetic layer, a nonmagnetic electrically conductive layer, a pinned magnetic layer and a second antiferromagnetic layer are stacked on the first antiferromagnetic layers and a portion on the track width one after another in the order the layers are enumerated. Since the free magnetic layer is created after the first antiferromagnetic layer, the free magnetic layer and the first antiferromagnetic layer are adhered to each other with a high degree of reliability. When the direction of magnetization in the free magnetic layer is changed by an external magnetic field, the electrical resistance of the magnetoresistive sensor also changes. The change in electrical resistance is, in turn, used for detecting the external magnetic field. Since the first antiferromagnetic layers put the free magnetic layer in a single-domain state in the X direction, the amount of Barkhausen noise can be reduced.