A signal processing circuit capable of preventing illicit copies, preventing the circuit from being unable to discriminate a plurality of cipher mode and unable to decipher, and correctly decipher received data at the reception side, configured to be provided with a cipher mode continuity discrimination circuit which confirms the continuity of a cipher mode at the time of reading transmission data from an FIFO when transmitting a plurality of packets, stops the transmission when a discontinuity is confirmed even if there is room in a band enabling transmission in the transmission cycle of the 1394 standard, and instructs a link core to transmit the packet enciphered by a different cipher key at the next cycle, so that only the data enciphered by one cipher mode is transmitted within one cycle of the 1394 standards, and the data enciphered by a different cipher mode is transmitted in the next cycle.