The on-board device of this portable terminal authentication system transmits a signal to a vehicle-side transmission unit, said signal including preset multiple authentication signals having different reception signal strengths. The portable terminal device instructs a portable-terminal-side reception unit to use a reception sensitivity characteristic determined on the basis of the strength values of the received authentication signals, and discriminates between the received multiple authentication signals after the reception sensitivity characteristic of the portable-terminal-side reception unit is adjusted to match the instructed reception sensitivity characteristic. A vehicle-side control unit or a portable-terminal-side control unit determines, on the basis of the discrimination result of the multiple authentication signals, whether or not the portable-terminal-side reception unit received a legitimate request signal during the communication.
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