A quality indicator is a measure of medical service quality. To develop quality indicators that are useful in fair comparison of medical service quality in multiple hospitals, it is helpful to develop them in a proper representation framework. The authors introduce a representation system of quality indicators and show the usefulness of the system by using existing quality indicators. To this end, they organize problems of development of quality indicators from the viewpoint of knowledge science, and analyze expression patterns of assessment criteria of medical services. This paper shows that the representation system has a certain amount of ability to address the problems above, and sufficient power of expression to describe assessment criteria including quality indicators in the patterns above.