Abstract: Serially transmitted digital signal train is divided into frames and each frame synchronous code signal is given to each frame and plural frames are grouped to form blocks and each block synchronous code signal is given to each block to transmit the data. The first and second frame synchronous code signals having a small cross correlation function in the same code length are used in said transmission system. The first frame synchronous code signal is used only for the top frame of the block and the second frame synchronous code signal is used for the other frames. The first frame synchronous code signal is used for both of the frame synchronism and the block synchronism whereby it is unnecessary to use different block synchronous code signals to prevent the increase of transmission redundancy and to increase transmission efficiency.This application is a continuation of application Ser. No. 471,603, filed Mar. 8, 1983 which is a continuation of application Ser. No. 137,805 filed Apr. 4, 1980.