To avoid the deterioration of service quality by detecting the silent fault of a base station.

A fault detection method of one embodiment includes: a step for allowing a radio terminal 100 corresponding to multiple radio access systems to transmit failure notification information indicating that connection to a base station 200A is failed to a base station 200B, which is included in a radio access network 10B with a radio access system different from that of a radio access network 10A, after connection to the base station 200A included in the radio access network 10A is failed; a step for allowing the base station 200B to transmit alarm information to a monitoring device 300B when receiving the failure notification information from the radio terminal 100; and a step for allowing the monitoring device 300B to detect the silent fault of the base station 200A on the basis of the alarm information from the base station 200B.