Truth Maintenance System (TMS), proposed by Doyle, is a database system which incorporates default logic and non-monotonic reasoning. Major facilities of TMS are (1) Truth Maintenance (TM) which propagate changes introduced into the database, and (2) Dependency Directed Backtracking (DDB) which resolves a contradiction by inverting appropriate assumptions within the database. However, TMS is written in conventional procedural language (Lisp), and its function cannot be extended or modified easily. So the authors have developed a system which is functionally equivalent to TMS, using concurrent object-oriented programming language ABCL/1. The system is a collection of ABCL/1 object definitions, and the system can be easily extended by adding necessary object classes. Moreover, TM/DDB algorithms are totally revised to make full use of parallelism inherent in the TM/DDB process. The TM algorithm allows multiple truth maintenance process to run concurrently, with effective elimination of duplicate processing between them. The DDB algorithm uses parallel search to locate appropriate invertible assumption set, and allows choices between various search strategies, in user-modifiable manner. As a whole, use of object-oriented concurrent programming language led to simple, readable and natural description of the system. The authors are planning to use the system as a basis for future research of truth maintenance algorithms and development of actual knowledge-based applications.