The present invention provides a data input/output control device integrating a one-chip microcomputer together with a data transfer device and a processor, the data transfer device being constructed to transmit to and receive from an external apparatus serial data and the processor processing data inputted to the data transfer device and transmitting the processed data to the data transfer device to be further transmitted to the external apparatus, the date input/output control device characterized in that a clock of the data input/output control device for an operation thereof is a transfer clock utilized by the external apparatus and the transfer clock is slower than a clock for the processor.The data input/output control device comprises a controller for controlling the data input and the data output of the data transfer device, a flag holding unit for holding a flag which shows if the data have been inputted to the data transfer device by the processor or the external apparatus, a first synchronization circuit for synchronizing an output from the flag holding unit with the transfer clock, the output being sent to the controller, and a second synchronization circuit for synchronizing the output from the flag holding unit with the clock for the processor.