This paper discusses the merits and shortcomings of the estimation methods previously used for the degree and mass of magma degassing in the volcanic eruption. On the basis of these methods, the authors propose a new petrological method for the estimation performed at various stages in the process of volcanism, and illustrate the detailed steps as exemplified by the Jingpohu Holocene volcanic eruption. This example shows that the degrees of degassing are changeable in the eruption, and that the major volatiles and their released mass cannot be inferred in the light of their relative abundances in glass inclusions. Finally, the possible application of this new method is discussed.