PURPOSE: To reduce the cost of a transmission circuit by using the transmission circuit as a pair in each relaying spot, and connecting the respective transmission circuits by transmission circuits of plural routes.

CONSTITUTION: This transmission device is constituted of plural pairs of transmission circuits Cn, C'n provided at every relaying spot separated from each other, and transmission lines Ln, L'n of two routes which become one pair for connecting each relaying spot. In such a state, each first system transmission circuit Cn of each relaying spot corresponds to reception of forward direction data through a first line Ln-1 from a first system transmission circuit Cn-1 of the upstream side, and transmits immediately the latest reverse direction data obtained already from the repeating spot of the downstream side equally to a first system transmission circuit Cn-1 and a second system transmission circuit C'n-1 of the relaying spot of the upstream side through a first line Ln-1 of the upstream side. By keeping pace with this operation, a first system transmission line Cn transmits equally the forward direction data from the upstream to a first system transmission circuit and a second system transmission circuit so as to become a prescribed cycle in accordance with reception of the reverse direction data through a first circuit Ln from a first system transmission line Cn+1 of the downstream side.