A variable rate transmission method that can vary the transmission rate of data. A transmitting side supplies a transmitted data sequence to an error detecting encoder (105) and a frame memory (103). The frame memory (103) stores data of a variable length to be transmitted in one frame. The error detecting encoder (105) calculates an error detecting code (such as CRC code) for each frame of the transmitted data. A multiplexer (104) adds the calculated error detecting code ahead of the transmitted data to place it at the initial position of the frame, and sequentially outputs the data sequence frame by frame. A receiving side calculates an error detecting code of the data in each transmitted frame in the same manner as the transmitting side, and compares the calculated error detecting code with the error detecting code at the initial position of the frame. The end bit of the frame data is decided as a position at which the two error detecting codes coincide. This makes it possible to transmit variable length data without informing the receiving side of the data length in each frame. This is equivalent that the transmission rate can be varied freely. A variable rate transmission method that can vary the transmission rate of data. A transmitting side supplies a transmitted data sequence to an error detecting encoder (105) and a frame memory (103). The frame memory (103) stores data of a variable length to be transmitted in one frame. The error detecting encoder (105) calculates an error detecting code (such as CRC code) for each frame of the transmitted data. A multiplexer (104) adds the calculated error detecting code ahead of the transmitted data to place it at the initial position of the frame, and sequentially outputs the data sequence frame by frame. A receiving side calculates an error detecting code of the data in each transmitted frame in the same manner as the transmitting side, and compares the calculated error detecting code with the error detecting code at the initial position of the frame. The end bit of the frame data is decided as a position at which the two error detecting codes coincide. This makes it possible to transmit variable length data without informing the receiving side of the data length in each frame. This is equivalent that the transmission rate can be varied freely.