PURPOSE: To confirm the path of a data conversion part while utilizing data on a line by providing an elastic store memory for storing the data to be inputted to a conversion input part for the delay time of the conversion part, and comparing the output of the conversion part with the output of the elastic store memory.

CONSTITUTION: The elastic store memory 2 is provided to store the data for the delay time of an in-device conversion part 1 while utilizing the data on the real line on a frame to be inputted to the in-device conversion part 1, and the output of the in-device conversion part 1 is compared with the output of the elastic store memory 2. Since the phase of the data conversion part in a multiple converter is synchronized to an office clock without fail and the length of the phase before and behind the data conversion part 1 is fixed, phase shift or the generation of error caused by asynchronism does not occur even when the input part data of the data conversion part 1 are written into the elastic store memory 2 and read out while delaying the fixed length and the output is compared with the output of the data conversion part. Thus, no influence is generated even while operating the line and in-device diagnosis can be executed even without an idle time slot for a test.