In a communication apparatus performing communication using IP packets, a diagnostic frame terminator terminates a series of latest status information of diagnosed devices recorded in a diagnostic frame every time the diagnostic frame sequentially passes through a series of diagnosed devices connected in cascade; a content addressable memory preliminarily stores therein as a single entry a series of normal status information in which each status information of the series of diagnosed devices in a normal state is arranged in an order of connection of the diagnosed devices, and generates an abnormality notification when the terminated series of latest status information does not hit the entry; a status information storage stores the terminated series of latest status information; and a CPU acquires, upon receipt of the abnormality notification, the terminated series of the latest status information from the status information storage to be outputted.