PURPOSE: To easily synchronize a system and the MODEM device by providing a buffer control part for changing and setting the amount of data to be stored in a buffer corresponding to communication speed so that the cycle of performing access to the system can be almost fixed.

CONSTITUTION: A buffer control part 3 designates the buffer to be driven in buffers 11-1n corresponding to the communication speed so that the access cycle to the system can be almost fixed. Namely, the amount of data to be stored in the buffer group 1 is changed and set corresponding to the communication speed so that the cycle of performing access to the system can be almost fixed. Since the access to the system is performed in the almost fixed cycle even when the communication speed is changed, the signal of this almost fixed cycle is inputted to a function block to be timer-controlled on the system to which the MODEM device is connected. Thus, the system and the MODEM device can be easily synchronized.