PURPOSE: To switch a transmission line and to detect interruption of the transmission line in a short time by switching the transmission line depending on the quantity of a data number detected by 1st and 2nd data number detection sections.

CONSTITUTION: A data is extracted from transmission lines I, II by the 1st and 2nd data number detection sections 20, 21, which count its data number and store the result. A switching control circuit 5 compares data number outputted from the 1st and 2nd data number detection sections 20, 21 to control a selector 9, which selects a transmission line having more data number. A timer 6 synchronizes data sample circuits 2a, 2b of the 1st and 2nd data number detection sections 20, 21, data number counters 3a, 3b, status latch circuits 4a, 4b and a switching control circuit 5 at a maximum delay time interval of data of the 1st transmission line 1 and a data of the 2nd transmission line II.