PURPOSE: To enable easily the transmission of data between host and subordinate devices without being conscious of the control of transmission and the basic transmission procedure, by providing a transmission buffer to a controller to ensure a unified control of continuous input and output processes.

CONSTITUTION: A computer 1 is connected to a process input/output device 3 via a buffer 7 of controller 2. The computer 1 performs an input/output control and an arithmetic process of each data to a logic address of the buffer 7 which is previously decided. Then a control task 8 between the outside and an application within the controller 2 is executed without being conscious of the basic transmission procedure which is based on a data transmission format. As a result, the mixture of errors can be prevented in a forming process of the transmission procedure at the application side. This can easily support the development of a program.