The present invention provides a manufacturing method of a magneto-resistive element capable of obtaining a higher MR ratio, in a method of forming a metal oxide layer (e.g., MgO layer) by oxidation treatment of a metal layer (e.g., Mg layer). An embodiment of the present invention includes the steps of; providing a substrate having a first ferromagnetic layer; fabricating a tunnel barrier layer on the first ferromagnetic layer; and forming a second ferromagnetic layer on the tunnel barrier layer. The step of fabricating the tunnel barrier layer includes; the steps of; depositing a first metal layer on the first ferromagnetic layer; oxidizing the first metal layer; depositing a second metal layer on the oxidized first metal layer; and performing heating treatment on the oxidized first metal layer and the second metal layer at a temperature at which the second metal layer boils.