A behavior of an embedded system is designed by a state machine, which receives input events from externals through the interface, and sends output actions to externals through the interface. Thus the interface specification becomes major concerns in the system design. This paper describes a design method that defines the interface of a component by protocol state machines, and then systematically constructs a behavioral state machine of the component using the interface protocol state machines. The design support system developed to realize the method, which provides enough guidance for designing behavior state machine by using protocol specification, is also described.