A magnetic body device has a stacked structure comprising an underlying layer, a magnetic body layer, and a cap layer. The material for the underlying layer is different from that for the cap layer. The magnetic body layer has a free magnetization region having perpendicular magnetic anisotropy and a first characteristic change region and a second characteristic change region situated on both sides of the free magnetization region in a first in-plane direction. The perpendicular magnetic anisotropy of the first characteristic change region and the second characteristic change region is at a level lower than that of the free magnetization region. An external magnetic field containing a component in the first in-plane direction is applied to the free magnetization region. Further, a current in the first in-plane direction is supplied to the free magnetization region.