Supplied with first and second signals from first and second control units (19, 29), first and second reading units (17, 27).read first and second readout ATM signals from first and second primary memory units (15, 25) which memorize an ATM signal (11). The second control unit produces the second-reading signal when a predetermined interval lapses after supplied with a first reading start signal from the first control unit. A first processing unit (21) processes the first readout ATM signal into a first STM signal and a first processed content signal. A second processing unit (31) processes, in response to the first processed content signal, the second readout ATM signal into a second STM signal. An output reading unit (35) read first and second output STM signals at the same time from first and second secondary memory units (23, 33) which memorize the first and the second readout STM signals.
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