The state of plant operation in terms of safety, product quality and productivity can be found by grasping semi-quantitatively the excess or deficiency in performance of equipment provided by maintenance teams relative to the performance required by production teams. In grasping excess or deficiency, it is necessary to develop a means by which production and maintenance teams can reach a mutual understanding of the reliability that each expects of equipment. The reliability of equipment cannot be evaluated solely by scientifically proven facts, but requires judgment based on people's overall understanding, which includes assumptions based on experience and conjecture. To understand the level of reliability in such a comparable form, it will be possible by this means to grasp systematically and semi-quantitatively the effects of both teams on equipment operation as a level of latent risk, and also to evaluate objectively the overall reliability of equipment during operation by establishing appropriate indices.