A main unit of a key telephone system accommodates a plurality of terminal units of the key telephone system that use a LAN within company as extension lines and exchange calls with outside lines. A device for the secure protection of a portion of the surface of an official document and such an official document are also described. A light transmitting rubber sheet 11 closes a space 30 surrounded by the rib 7 and is held between a circuit board 15 and the rib 7. The furfural-containing gaseous stream is recirculated to bring it into further contact with the heated solid biomass to enrich the concentration of furfural. Compiled activity data represents summary statistical information that is created by considering some or all of the event data.