Making use of a generalized master equation, it is shown that the proposed description of the macroscopic nature leads to decoherence under certain condition. The decoherence parameter is explicitly discussed. Starting from the Machida-Namiki theory of measurement based on the many Hilbert spaces, I) an alternative description of the macroscopic nature has been developed. 2 l, 3 l This description was applied to a macroscopic system 2 l and a quantum mechanical measuring process. 3 l The criterion for fundamental division into quan tum and classical system also was given in these works. It is an important problem how, under what condition, the transition from quantum to classical system occurs as a real physical process. 4 l- 9 l The relation between quantum and classical mechanics is one of the basic problems in the theory of measurement, interpretation of quantum theory and for the construction of objective quantum physics. 4 ),s) Although to con sider the macroscopic body as a quantum open system is common to these theories which intend to explain the state reduction, each theory follows different lines and has widely different outward appearances. Macroscopic system as an open system with the uncertainty of the number of atoms induces the reduction of wave packet. 1 l