A signal adapter box for use to extend the signal transmission distance between a signal source and a video output device is disclosed to include a housing, which has a first opening and a second opening for the insertion of a HDMI plug and a RJ-type plug respectively, a circuit board mounted in the housing, a HDMI jack mounted on the circuit board and abutted against the first opening of the housing for receiving a HDMI plug of a HDMI cable, and RJ-type jacks mounted on the circuit board for signal transmission with the HDMI jack and abutted against the second opening of the housing for receiving a RJ-type plug of a RJ-type transmission cable respectively.