To provide a branch line branching device that can quickly repeat a signal between a trunk and a branch line with a simple structure.

Buffer circuits 31 and 32 for transmitting a signal from a trunk to a branch line between trunk side terminals 11 and 12 and branch line side terminals 21 and 22 and buffer circuits 33 and 34 for transmitting the signal from the branch line to the trunk are connected. Being based on whether or not there is a signal on the branch line side terminal 11, an intransmission judgment circuit 35 makes the buffer circuits 31 and 32 or the buffer circuits 33 and 34 active states or high impedance states.