A magnetoresistive effect sensor includes a pinned layer, a nonmagnetic spacer layer, and a free layer having a total thickness of at least 10 nm. The sensor has insulating films disposed on both side surfaces of the spin valve film. The sensor has hard magnetic films disposed on the insulating films for applying a biasing magnetic field to the free layer. Each hard magnetic film extends toward the free layer in a vicinity of the spin valve film, such that as each hard magnetic film extends toward the spin valve film, a cross-sectional area thereof in a plane perpendicular to the layer width direction becomes progressively smaller. Each hard magnetic film has, in a plane parallel to the air bearing surface, a first boundary line which at least partially faces the free layer and substantially defines an end point of the hard magnetic film in the layer width direction.