User identification system and vehicular portable device

A user identification system comprising:

• an in-vehicle apparatus mounted to a vehicle used by a plurality of users;
• and
• a vehicular portable device carried by a user acting as a driver among the users and associated with the in-vehicle apparatus,
• wherein:
• the in-vehicle apparatus includes
• a vehicle apparatus-side communicator that communicates with the vehicular portable device;
• the vehicular portable device includes:
• a first communicator that communicates with the in-vehicle apparatus;
• a second communicator that performs a short range wireless communication with portable terminals carried by the users;
• a received signal strength detector that detects received signal strength of signals received by the second communicator from the portable terminals;
• and
• a driver terminal determiner that, based on the received signal strength detected by the received signal strength detector, determines a driver terminal which is the portable terminal carried by the user acting as the driver;
• the received signal strength detector detects the received signal strength of the signals received from the portable terminals at a plurality of time points;
• and
• the driver terminal determiner determines as the driver terminal the portable terminal that provides a smallest degree of change of the received signal strength detected multiple times.