A network management apparatus in a mobile communication network holds a communication quality index in a normal operation and periodically receives input of a communication quality measurement result from a radio base station control apparatus. When a connection failure count f with respect to a connection request count a of each radio cell is obtained as a measurement result, letting p0 be the call loss rate in the normal operation, an upper probability B of a binomial distribution representing the connection failure count becomes larger than f is obtained (step 524). The negative logarithm of the upper probability B is obtained as the score of the degree of abnormality (step 525). Anomaly of communication is detected when the score of the degree of abnormality exceeds a predetermined threshold value (steps 526 and 527). After that, maintenance control is executed in accordance with the calculated score of the degree of abnormality, thereby appropriately avoiding a fault of the communication system. This allows to calculate the degree of abnormality from the measurement result of the communication quality index in the mobile communication network in consideration of the statistical reliability and execute maintenance corresponding to the degree of abnormality.