The present invention is directed to align crystal c-axes in magnetic layers near two opposed junction wall faces of a magnetoresistive element so as to be almost perpendicular to the junction wall faces. A magnetic sensor stack body has, on a substrate, a magnetoresistive element whose electric resistance fluctuates when a bias magnetic field is applied and, on sides of opposed junction wall faces of the magnetoresistive element, field regions including magnetic layers for applying the bias magnetic field to the element. The magnetoresistive element has at least a ferromagnetic stack on a part of an antiferromagnetic layer, and width of an uppermost face of the ferromagnetic stack along a direction in which the junction wall faces are opposed to each other is smaller than width of an uppermost face of the antiferromagnetic layer in the same direction.