 as described above, the thicknesses of the free magnetic layer and the fixed magnetic layers are decreased, and shunting of the sensing current can be prevented, whereby the rate of change in magnetoresistance can be increased. In the spin valve thin-film magnetic device according to the present invention, it is preferable that one of the fixed magnetic layers described above be composed of a first ferromagnetic layer and a second ferromagnetic layer with a first nonmagnetic layer provided therebetween, in which the thickness of the second ferromagnetic layer formed at a location closer to the free magnetic layer is larger than that of the first ferromagnetic layer, and that the other fixed magnetic layer be composed of a third ferromagnetic layer and a fourth ferromagnetic layer with a second nonmagnetic layer provided therebetween, in which the thickness of the third ferromagnetic layer formed at a location closer to the free magnetic layer is smaller than that of the f