A software development support tool for developing softwares of concurrent systems is suggested and implemented. For this purpose LSS(Life Cycle Support System) generator approach is combined with the restricted form of the modified Petri net. Using an LSS generator ISLDM/SEM, a specification tool is generated whose underlying methodology is the modified Petri net. To achieve this, it is necessary to see the modified Petri net in the point of Entity-Relationship view which is the basic model of LSS generator approach. The specification in the developed system description language is checked against incompleteness, inconsistency, and ambiguity. Petri net structures are extracted from the database in modified form. Then they are fed into the analyzer tool. The analyzer converts modifications into equivalent original Petri net structures and checks dynamic behaviors of the modeled system using the concept of the reachability tree.