Abstract: An internal timing information producing portion produces internal timing information IRTS1 to IRTS4 which is the timing information on a data reception clock by using a network clock C.sub.N and the data reception clock C.sub.RU. A calculator calculates the difference between the internal timing information IRTS1 to IRTS4 and the transmission timing information RRTS1 to RRTS4 contained in a received cell, and a data reception clock producing portion adjusts the timing of the data reception clock C.sub.RU so that the difference is zero and outputs the adjusted timing. An ATM cell decomposing portion outputs the user data D.sub.RU in synchronism with the data reception clock C.sub.RU which is input from the data reception clock producing portion.