  Microcanonical analysis is a powerful method for studying phase transitions
of finite-size systems. This method has been used so far only for studying
phase transitions of equilibrium systems, which can be described by
microcanonical entropy. I show that it is possible to perform microcanonical
analysis of a nonequilibrium phase transition, by generalizing the concept of
microcanonical entropy. One-dimensional asymmetric diffusion process is studied
as an example where such a generalized entropy can be explicitly found, and the
microcanonical method is used to analyze a nonequilibrium phase transition of a
finite-size system.
