A signal transmission device disclosed in the present invention includes a casing, a first connector and a second connector. The connectors are disposed inside the casing. The first connector includes a first conductive terminal and a bridging component. The first conductive terminal is electrically connected to a socket of a first portable electronic device. The bridging component includes a conductive portion electrically connected to the first conductive terminal and a non-conductive portion. The second connector includes a conductive piece and a second conductive terminal. An end of the conductive piece contacts the conductive portion or the non-conductive portion according to position variation of the second connector. The second conductive terminal is electrically connected to the conductive piece, and can protrude from a hole on the casing when the conductive piece contacts the conductive portion, so as to insert into a socket of a second portable electronic device.