This paper introduces a Petri net model for a reason maintenance system and applies Petri net analysis methods to a finite nonmonotonic theory.The model proposed in this paper can not only specify the reason maintenance system, but also analyse the system based upon the net theory and techniques.It is important that the authors derive an algorithm to find the extensions for a given set of nonmonotonic rules and an algorithm to find out inconsistency in a system.Their algorithm to find extensions is considered to be more efficient than the existing reconstruction procedure, and it is decidable.