A radio communication module accommodated in an image information input/output
device capable of carrying out communication by varying a send output power to
a plurality of mobile radio terminals located away from the image information input/output
device through arbitrary distances respectively. A distance calculation module
calculates a distance of each of the mobile radio terminals from a send-output
power value of the radio communication module which is making communication with
each of the mobile radio terminal. A mobile radio terminal control module manages
identification information and user information specifying each the mobile radio
terminal, and the distances and time calculated by the distance calculation module.
An interval timer periodically makes communication with each the mobile radio terminal
and calculates the distance. Whenever the communication with each the mobile radio
terminal is made, a user judging module detects that any of the users approached
the image information input/output device based on the distance calculated by the
distance calculation module, and specifies a user based on the identification information
which specifies each of the mobile radio terminals in the plurality of mobile radio
terminals which are managed by the mobile radio terminal control module.