A data transmission apparatus which is connected with a shift register of a plural stages forming a forward path of data transmission line, and a shift register of a plural stages forming a backward path of the same, and a loop-back part therebetween, and comprises bypasses between the shift register on the forward path and on the backward path to bypass the transmitted data when significant data does not exist on the loop-back part side from a stage on which the bypass is comprised and no data stays at a stage where the bypass is comprised, so that data is transmitted at high speed, and which is constructed to be able to control the bypass from external, so that testing of circuitry is easy.