This paper presents a method of constituting a rock mass classification, which is capable of representing an engineer's subjectivity on engineering judgement and thought process by using the fuzzy set theory. The method is composed of three parts; (1) description of judging classification parameters by fuzzy sets, (2) grading of importance of each parameter, (3) total evaluation of judgement of all parameters by fuzzy integral. The classification provides a distribution of “fuzzy expected values” covering all the rock mass classes (five classes, i. e. “very good”, “good”, “fair”, “poor” and “very poor”, are adopted in this paper). An example is shown to illustrate the constitution of rock mass classification by the proposed method.