Most requirements elicitation methods do not explicitly provide a systematic way for deciding the boundary of the usage context that should be taken into account because it is essentially difficult to decide which context element should be included as the system requirements. If a developer explores the context boundary in an ad-hoc manner, the developer will be faced with the frame problem because there are unlimited context elements in the real world where the target system exists. There are many application domains that should take into account the frame problem: security, safety, network threats, and user interactions. To deal with this problem, this paper proposes a new type of requirements analysis method for exploring the context boundary using guide words, a set of hint words for finding a context element affecting the system behavior. The target of our method is embedded systems that can be abstracted as a sensor-and-actuator machine exchanging the physical value between a system and its context. In our method, only the value-context elements, a kind of value objects, are extracted as the associated context elements. By applying the guide words, we can explore only a sequence of context elements affecting the data value and avoid falling into the frame problem at the requirements analysis phase.