A position determining device is configured to determine the position of a portable device configured to receive one or more radio signals transmitted from one or more antennas inside a vehicle, respectively. The position determining device includes a processor configured to determine the open or closed state of a door of the vehicle for getting in and out of the vehicle, calculate an evaluation value related to the position of the portable device based on the received signal strength of the one or more radio signals at the portable device, and determine that the portable device is positioned inside the vehicle when the evaluation value satisfies a predetermined condition. The processor is configured to change at least one of the predetermined condition and a method of calculating the evaluation value in accordance with the determined open or closed state of the door.