Recently, researches for knowledge system development environment which supports advanced knowledge representations and reasonings have been progressing. The purpose of the WORLDS research is to develop one of these environment, which equips the multiple viewed-worlds mechanism for an assumption-based reasoning in incomplete knowledge environment. This paper presents the design concept and implementation of the knowledge system/language WORLDS and its simple application. The main features of WORLDS are summarized in following three points. 1) It has a focus of attention control mechanism, which attends to a subset of rules and a subset of objects. This mechanism constructs a hierarchical working memory between viewed-worlds and a knowledge base or an assumption set by adding premises from a knowledge base to viewed-worlds and generating assumptions from an assumption set successively. 2) It has a multiple viewed-worlds mechanism, which extends viewed-worlds based on a focus of attention control, monitors inconsistency among viewed-world nodes based on nogood relations and maintains viewed-worlds based on truth maintenance. 3) It has an amalgamated knowledge representation and inference method of object-oriented, rule-oriented and logic-oriented forms, which is planned to be suitable for an assumption-based reasoning by multiple viewed-worlds mechanism. This amalgamation is accomplished by logical interpretation of message passing and rule execution based on message passing. A simple knowledge base system for city planning support has been built using WORLDS's knowledge representation and reasoning facilities, and its ability has been confirmed.