A user identification system is provided. In the user identification system, an in-vehicle apparatus specifies a position of a vehicular portable device with reference to a vehicle and determines whether the vehicular portable device exists in a predetermined identification area. Based on determining that the vehicular portable device exists in the identification area, the in-vehicle apparatus transmits an instruction signal instructing the vehicular portable device to specify a driver terminal. Based on reception signal strength detected during a time period determined by a time point of receipt of the instruction signal, the vehicular portable device determines the driver terminal.