A data distribution system transmitting and receiving a large capacity of data such as image data smoothly without incurring a delay over an optical fiber line located between a data sever and a client device. In the data transmission system, a plurality of client devices are bus-connected through an optical fiber to a data server transmitting and receiving data as signal light. Each of the client devices has a client optical switch and is connected with a client access device. The client optical switch is structured with a thermal-lens forming element and a signal-light path shifting member. When any of the client devices forwards an access command to its own client access device, control light is irradiated from the client access device to the own client optical switch thus effecting a switchover. Thus, transmission data of from the data server is stored in a data storage of the relevant client device.