  A system-size expansion method is incorporated into the Doi-Peliti formalism
for stochastic chemical kinetics. The basic idea of the incorporation is to
introduce a new decomposition of unity associated with a so-called Cole-Hopf
transformation. This approach elucidates a relationship between two different
Langevin equations; one is associated with a coherent-state path-integral
expression and the other describes density fluctuations. A simple reaction
scheme $X \rightleftarrows X+X$ is investigated as an illustrative example.
