A magnetoresistance effect element comprising: a first ferromagnetic metal layer; a second ferromagnetic metal layer; and a tunnel barrier layer that is sandwiched between the first and second ferromagnetic metal layers, wherein the tunnel barrier layer is expressed by a chemical formula of AB 2 O x (0<x≤4), and has a spinel structure in which cations are arranged in a disordered manner, A represents a divalent cation that is either Mg or Zn, B represents a trivalent cation that includes a plurality of elements selected from the group consisting of Al, Ga, and In, characterized in that the tunnel barrier layer comprises: a lattice-matched portion that is lattice-matched with both of the first ferromagnetic metal layer and the second ferromagnetic metal layer; and a lattice-mismatched portion that is not lattice-matched with at least one of the first ferromagnetic metal layer and the second ferromagnetic metal layer, and when viewed as an inverse Fourier transform image in a stacking direction of a cross-sectional crystal lattice image of the interface between the tunnel barrier layer and the first and/or the second ferromagnetic metal layer, a lattice-matched portion is made up of a plurality of sequential, continuously-connected lattice lines, and a lattice-mismatched portion is made up of a plurality of sequential, non-continuously-connected lattice lines and/or no lattice line. a first ferromagnetic metal layer; a second ferromagnetic metal layer; and a tunnel barrier layer that is sandwiched between the first and second ferromagnetic metal layers, wherein the tunnel barrier layer is expressed by a chemical formula of AB 2 O x (0<x≤4), and has a spinel structure in which cations are arranged in a disordered manner, A represents a divalent cation that is either Mg or Zn, B represents a trivalent cation that includes a plurality of elements selected from the group consisting of Al, Ga, and In, characterized in that the tunnel barrier layer comprises: a lattice-matched portion that is lattice-matched with both of the first ferromagnetic metal layer and the second ferromagnetic metal layer; and a lattice-mismatched portion that is not lattice-matched with at least one of the first ferromagnetic metal layer and the second ferromagnetic metal layer, and when viewed as an inverse Fourier transform image in a stacking direction of a cross-sectional crystal lattice image of the interface between the tunnel barrier layer and the first and/or the second ferromagnetic metal layer, a lattice-matched portion is made up of a plurality of sequential, continuously-connected lattice lines, and a lattice-mismatched portion is made up of a plurality of sequential, non-continuously-connected lattice lines and/or no lattice line. characterized in that the tunnel barrier layer comprises: a lattice-matched portion that is lattice-matched with both of the first ferromagnetic metal layer and the second ferromagnetic metal layer; and a lattice-mismatched portion that is not lattice-matched with at least one of the first ferromagnetic metal layer and the second ferromagnetic metal layer, and when viewed as an inverse Fourier transform image in a stacking direction of a cross-sectional crystal lattice image of the interface between the tunnel barrier layer and the first and/or the second ferromagnetic metal layer, a lattice-matched portion is made up of a plurality of sequential, continuously-connected lattice lines, and a lattice-mismatched portion is made up of a plurality of sequential, non-continuously-connected lattice lines and/or no lattice line.