a network; and

at least two terminal units connected thereto,

wherein each terminal unit comprises:

session controlling means for controlling a session for enabling transmission/receiving of voice, image, and handwritten data to/from a remote terminal unit individually;

display means for displaying said image and said handwritten data,

wherein said image data and said handwritten data are overlapped and displayed on a display of said display means;

an image/handwritten data managing means for managing image/handwritten data,

wherein the image/handwritten data managing means has a plurality of planes,

wherein the managing means displays basic image data on one of the plurality of planes, the one of the plurality of planes being an image data plane, and displays handwritten data currently handled in communication on a different plane, the different plane being a handwritten data plane, so that image and handwritten data are displayed so as to overlap each other by putting the different planes in layers,

wherein the session controlling means comprises means for starting and ending voice communication, image communication and handwritten data communication independently, and

wherein said means for starting and ending voice communication, image communication and handwritten data communication independently is configured to make at least one of the image communication, handwritten data communication, and voice communication at a high quality, and end other communications even when a through-put of the terminal unit or a number of communication bands is insufficient,

wherein each terminal unit further comprises:

erasing/information transmitting means for erasing image and handwritten data from the display means through the image/handwritten data managing means and transmitting erasure information to the remote terminal unit,

wherein in the terminal unit, the erasing/information transmitting means can select either image or handwritten data or both of image and handwritten data as an object to be erased and erase a selected object from the display means; and

an erasing/information receiving means for erasing image and handwritten data from the display means through the image/handwritten data managing means according to the erasure information from the remote terminal unit,

wherein in the terminal unit, the erasing/information transmitting means notifies the remote terminal unit of completion of the object erasure in return for the erasure information so that the remote terminal erases the object from its display means according to the notice.