In a data transmission system for a camera, a central processing unit (CPU) for controlling operation of the camera communicates with various circuit arrangements using data signals in a serial form under control of a clock signal selected in dependence upon which the circuit arrangement is the communication designation of the CPU at the time. Some of the circuit arrangements are provided in the camera while the other of the circuit arrangements are provided in accessories attachable to the camera. The CPU communicates with the circuit arrangements provided in the camera under control of a clock signal having a higher frequency for increasing the data communication speed while it communicates with the circuit arrangements provided in the accessories under control of a clock signal having a slower frequency for improving the accuracy of the data communication.