PURPOSE: To attain flow control of a transmission data while the increase in the overhead is supprssed by providing a count means counting number of arrival of a busy signal over a prescribecd time, a comparison means comparing an output of the count means with a reference value and a means setting the period of a clock signal based on the output of the comparison means.

CONSTITUTION: A count meand 2 counts a busy signal every time the signal is received. After a specified time elapses from the start of busy signal reception, the count of the count means 2 is compared with a predetermined count by a comparison means 4. When the count of the counter means 2 is larger than the predetermined count, a transmission clock speed is lowered, and in other cases, the transmission clock speed in increased. For example, when the present count of the count circuit 2 is larger than the count comparison in the comparator circuit 4, the content of a transmission clock selection register 5 is set to a binary number '01', the transmission clock A line 101 is switched, a transmission clock B line 102 is selected to select the transmission clock having a slower speed than tht of the transmission clock.