Abstract: Herein disclosed is a serial data transfer system which has first and second serial data processors connected via a single data line and a single clock line for transferring serial data therebetween. Each of the first and second serial data processors includes: reception confirmation signal output means for outputting a reception confirmation signal to the data line; and reception confirmation signal detection means for detecting the reception confirmation signal on the data line. The confirmation of the data transfer is executed in synchronism with serial clock pulses outputted to the clock line. Alternatively, the first or second serial data processor includes: an output circuit for outputting a reception confirmation signal to the data line; a circuit for generating a first signal indicating the end of reception of the serial data; a circuit for generating a second signal indicating the end of processing of the data received; and a circuit for controlling the output of said reception confirmation signal.