PURPOSE: To prevent the instantaneous interruption of data in a transmission switching state and to improve the maintenance performance and the reliability of a deivce by switching the transmission lines after aligning the positions in the transmission frames of two data to be switched by a panel memory circuit and sending those data to a switching circuit.

CONSTITUTION: 1st and 2nd memory circuits 32 and 33 receive one of two data floating the data frames of the same contents on the transmission frames respectively and designating the head positions of both data frames with a pointer and then write the data frames at each head position according to the designation of the pointer. A control circuit 34 applies the control signals to both circuits 32 and 33 to instruct them to read out the data frames in the same read timing. Then a switching circuit 38 selects and outputs one of both data read out of the circuits 32 and 33.