The event message is transmitted from an operative event detection device to the application, the transmission history is held by a device different from the operative event detection device, an event identifier of an event message and a time stamp in the generation history held by a standby event detection device is compared with an event identifier of an event message and a time stamp in the transmission history when the standby event detection device detects an abnormal condition of the operative event detection device, thereby transmitting an event message that has not been transmitted by the operative event detection device through the standby event detection device, and allowing the standby event detection device to transfer from the standby event detection device to an operative system.