To provide a data transmission system where a receiver can securely obtain synchronism with a signal frame transmitted at the head of a call.

In this data transmission system, a transmitter sends a signal by a transmission frame constituted of a cipher code for synchronism, which follows a unique word UW6, and of a data code following the unique word led by the cipher code. A receiver detects the unique word prior to the cipher code, detects the cipher code for synchronism, and synchronizes it with the transmission frame. The transmitter adds preamble unit words UW1 to UW5 different from the unique word before the transmission frame. The receiver receives the signal by synchronizing it with the transmission frame based on the unique word whose detection is detected to the most secure among the unique words prior to the cipher code.