An object of the invention is to provide an adaptor device for a car radio capable of improving the receiving state. The adaptor device 20 for a car radio is connected to a vehicle-mounted antenna connection terminal 12 provided in an FM radio receiver 10 installed within a vehicle room, and includes a signal line inserted between the vehicle-mounted antenna connection terminal 12 and a vehicle-mounted antenna 110; and an in-vehicle antenna 22 branching out from the signal line. A radio wave transmitted from an FM transmitter 40 disposed within the vehicle room via an antenna 42 is received by the radio receiver 10 via the in-vehicle antenna 22.