PURPOSE: To always transfer key data and to improve the efficiency of data transfer between a keyboard and a computer, by separating key data from page/ book data in keying information, and transferring the page and book data only when these data are changed.

CONSTITUTION: A serial interface transmission line 3 is used to connect the body 1 of a computer with a book type keyboard 2, and the keyboard 2 is provided with a keyboard controller 21 and a key matrix 22. The body 1 applies polling with a fixed period and a prescribed format to the keyboard 2. Receiving the polling, the keyboard 2 sends the 1st data including a request bit indicating the kind of the data. In addition, the keyboard 2 sends the 2nd data consisting of status data including a bit indicating the switching of page and book data, key data and page/book data in response to the polling data. Consequently, the efficiency of data transfer between the computer 1 and the keyboard 2 is improved.