There is provided a method for controlling the quality of a product, characterized in that a product and an oxygen detecting agent having a predetermined sensitivity corresponding to the kind of the product and quality control conditions for the product are provided in combination and the quality of the product is controlled by taking advantage of such a phenomenon that, when a predetermined period of time corresponding to the kind of the product and the quality control conditions for the product has elapsed, the oxygen detecting agent undergoes a change in color as a result of the detection of the concentration of oxygen, which has infiltrated the oxygen detecting agent, by the oxygen detecting agent. There is also provided an oxygen detecting agent characterized by comprising an oxygen detecting agent body and a film that hermetically covers said body and has a predetermined oxygen permeability and/or carbon dioxide permeability corresponding to the kind of a product, of which the quality is to be controlled, and the quality control conditions for the product.