The present invention is directed to determine validity of input data without increasing the number of data lines. An input/output system has an output device and an input device. The output device multiplexes data obtained by latching original data at a rising edge of a clock signal and data obtained by latching the original data at a falling edge of the clock signal and inverting the latched data, outputs the resultant data as multiplexed data, and also outputs the clock signal. The input device determines validity of the multiplexed data by comparing data obtained by latching the multiplexed data at a rising edge of the clock signal and data obtained by latching the multiplexed data at a falling edge of the clock signal and inverting the latched data.