A magnetoresistance effect device includes: a magnetoresistance effect element formed by performing lamination such that a spacer layer is disposed between a first ferromagnetic layer and a second ferromagnetic layer; a high frequency signal line arranged on one side of the magnetoresistance effect element in a direction parallel to a lamination direction; and a magnetic member arranged at a position further away from the one side than the high frequency signal line when viewed from the magnetoresistance effect element, wherein the magnetic member has a concave portion which is recessed in a direction away from the high frequency signal line in a surface facing the high frequency signal line.