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.