The present invention provides a presumable operator determining system which allows determining, at minimum cost, a presumable operator who has operated a door or a button of a home appliance or operated the home appliance by voice control. A presumable operator determining system (100), which determines a presumable operator who has operated a home appliance (10) provided in a house (40), has pre-registered therein unique information of respective communication terminal devices (20) such that the unique information are respectively associated with users who operate the home appliance (10), the communication terminal devices (20) being respectively carried by the users and brought along by the users when the users go out. In a case where the home appliance (10) is operated, the presumable operator determining system (100) (i) searches, with use of a communication section included in the home appliance (10), for a communication terminal device (20) connected to a relay station (30) for wireless LAN in the house (40) among registered communication terminal devices (20) and (ii) determines a presumable operator among a user(s) associated with a terminal device(s) (20) detected through the search.