A method of transmitting a variable-length frame by which synchronization can be established correctly and easily on the receiver side without causing frames to be redundant even in an environment where code errors are liable to occur. A variable-length frame division section (1) of a transmitter divides a variable-length frame (f) into a code sequence (f1) and a code sequence (f2) at a length ratio of 1:1. A first synchronizing word addition section (3-1) adds a synchronizing word (S1) to the head of the sequence (f1), and a second synchronizing word addition section (3-2) adds a synchronizing word (S2) to the head of the sequence (f2). Although differing in content, the synchronizing words have the same length. The code sequences to which the synchronizing words have been added are multiplexed by a selector switch (4) and built into a variable-length frame. A series of variable-length frames thus generated by the switch (4) are sent to a receiver as serial data. On the receiver side the start and end points of each frame are detected based on the positions at which the synchronizing words in the serial data are detected.