A position determination system determines a position of a mobile terminal relative to a vehicle by a wireless communication. The position determination system includes: a communication device disposed in a vehicle compartment and configured to receive a wireless signal transmitted from the mobile terminal and detect a reception strength of the received wireless signal. The position determination system is configured to: determine whether the mobile terminal is existing in the vehicle compartment; estimate an amount of absorbers that exists in the vehicle compartment and may absorb radio waves having a frequency used for the wireless communication; adjust an inside determination value which is used as a threshold for determining that the mobile terminal is existing in the vehicle compartment; and determine that the mobile terminal is existing in the vehicle compartment in response to the inside device strength being equal to or greater than the inside determination value.