An in-vehicle system is provided. A first transmitting antenna (23a) is disposed in a substantial center in a front portion inside a vehicle (101). A second transmitting antenna (23b) is disposed in the substantial center in a rear portion inside the vehicle (101). A third transmitting antenna (23c) is disposed in a side surface near a driver seat (111D) of the vehicle (101). A receiving antenna receives a response signal, the response signal being transmitted from a portable device (12) receiving the first signal transmitted from the first (23a) to third transmitting antennas (23c). A transmission controller controls existence or non-existence of the transmission of the response signal or interference signal from the first (23a) to third transmitting antennas (23c) and a transmission pattern that is a pattern of a transmission intensity. A position detector detects a position of the portable device based on a transmission content of the signal from the first to third transmitting antennas and a reception result of the response signal from the portable device. The in-vehicle system is, for example, applicable to systems for controlling vehicles.