A method of production of a magnetoresistance effect device is able to prevent or minimize a drop in the MR ratio and maintain the high performance of the magnetoresistance effect device even if forming an oxide layer as a surface-most layer constituting a protective layer by the oxidation process inevitably included in the process of production of microprocessing by dry etching performed in a vacuum. Two mask layers used for microprocessing are doubly piled up. This method of production of a magnetoresistivity effect device including a magnetic multilayer film including at least two magnetic layers includes a step of providing under a first mask material that is a nonorganic material a second mask material able to react with other atoms to form a conductive substance, and a device made according to the method.