PURPOSE: To lower the probability of the erroneous raising of an AIS alarm by adding the occurrence of synchronizing step-out to an AIS alarm raising condition.

CONSTITUTION: The synchronizing step-out (REC) is decided and in the absence of the REC, no processing relating to the AIS alarm is implemented. In the presence of the REC, the reception decision of the AIS signal is implemented continuously. When the absence of the reception of the AIS signal is discriminated, the AIS alarm is raised. Moreover, when the absence of the reception of the AIS signal is discriminated, the REC alarm is raised. The probability of erroneous raising of synchronizing step-out alarm is far lower than the probability of mistaking the AIS reception discrimination. Thus, the probability of the erroneous raising of the AIS alarm is lowered.