According to the present invention, a magnetoresistive sensor device, which can be manufactured at a high yield and a low cost, is excellent in magnetoresistance characteristics and is reliable can be provided. The magnetoresistive sensor device comprises a substrate, a signal processing circuit formed on the substrate, a flattening film for flattening the signal processing circuit, a silicon nitride film formed on the flattened signal processing circuit, and magnetoresistive sensor elements formed on the silicone nitride film, in which the flattening film is preferably a spin-on-glass (SOG) film.