PURPOSE: To detect a fault in a memory by generating a specific data from a check data generating means to a specific idle channel with respect to a memory storing conversion information for format conversion and using a check means to check an output read from the memory accordingly.

CONSTITUTION: Pilot data generating sections 15,17 are started in an idle time slot in a time slot of sender side and radio receiver side respectively and generates a preset check use pilot data and give it to memories 1, 2 of a memory section 14. Then storage/readout is applied at a part of storage location of the memories 1, 2, the data is sent to pilot check sections 16,18 and whether or not the read data is coincident with the preset data is collated. In case of dissidence, it is discriminated to be the occurrence of a memory error, then an alarm signal is generated to inform it to a supervision controller 5.