The operator's response to alarms is crucial in preventing a process upset from escalating into a more serious event in continuous processing in petrochemical plants. In evaluation of man/machine interaction during plant operation, EEMUA (Engineering Equipment and Material User Association) guidelines are best known as a standard based on the experience of many plant engineers. This paper employs a queuing system model for the alarm/operator process, and proposes an evaluation criterion calculated from the occupation rate using the queuing model. It is verified that the derived model substantiates the number of standing alarms in EEMUA guidelines. The method has the advantage that it can provide an operation margin to existing plants. Additionally, it is expected to be used in operation design when integrating control rooms.