An object of the present invention is to make it possible to easily evaluate silage fermentation quality on site or the like. In one embodiment of the present invention, a gas generated from silage is applied to a surface stress sensor, and the amount of either one of organic acids and nitrogen-containing compounds contained in the silage is determined. The surface stress sensor can detect trace components in a gas by a simple device configuration and in simple procedures. Therefore, by utilizing the fact that relationship between the content of these components and the fermentation quality is known, the evaluation of fermentation quality can be easily realized by the above measurement.

An object of the present invention is to make it possible to easily evaluate silage fermentation quality on site or the like. In one embodiment of the present invention, a gas generated from silage is applied to a surface stress sensor, and the amount of either one of organic acids and nitrogen-containing compounds contained in the silage is determined. The surface stress sensor can detect trace components in a gas by a simple device configuration and in simple procedures. Therefore, by utilizing the fact that relationship between the content of these components and the fermentation quality is known, the evaluation of fermentation quality can be easily realized by the above measurement.