Medical service needs two kinds of knowledge. One is the knowledge with high objectivity (e.g. care guidelines based on medical knowledge), and the other is vague knowledge (e.g. foresight on medical treatment sites based on assuming patient's psychology and difficulty. The author calls it practical knowledge). If we want to improve medical service continuously, how these two kinds of knowledge are shared becomes a problem. The author quests for a method to share the practical knowledge. The key idea of this method is to externalize the aims of medical services using an ontology (patient problem ontology). The externalized aims plainly express the service providers' intentions. In this paper, how to define the aims of medical services as ontology is explained, and a scenario to share medical service knowledge based on the ontology is shown.