A data transmission apparatus and a camera system for multiplexing serial data into horizontal blanking portions of video data for transmission between a CHU (1) and a CCU (2). On the transmitting side, resampled data is obtained by sampling the serial data using a clock signal of a frequency higher than that of the transmission rate of the serial data. A predetermined number of resampled data items are selected as multiplexing data from the resampled data obtained in each of the horizontal periods of the video data. The multiplexing data is multiplexed into the horizontal blanking portions of the video data for transmission. On the transmitting side, the multiplexing data is separated from the received video data. The separated multiplexing data is decoded so as to extract successively definite data constituting the initial serial data.