This paper presents a formal specification language for embedded network systems toward automatic test generation, named Sens. Embedded systems have become increasingly connected and communicated each other to form a large-scaled and complicated embedded network systems. In order to make design and testing of embedded network systems more reliable, we have started a collaborative project to develop an automatic test generation system based on a formal specification language. As the first step of the project, we have developed a formal specification language, Sens, whose features are property-based, test-oriented, object-oriented, and modular description toward efficient test generation for embedded network systems. In this paper, we give an important part of the syntax and semantics of Sens and the result of a feasibility study of specifying real industrial air-conditioning network systems using Sens.