An inter-vehicle transmission system comprising: a first transmission device disposed in a first vehicle including a first electrical coupler; and a second transmission device disposed in a second vehicle including a second electrical coupler electrically connected to the first electrical coupler, the second transmission device being capable of communicating with the first transmission device, wherein the first transmission device includes a first pair of terminals connected to the first electrical coupler via a first pair of signal lines, a transmission unit connected to the first pair of terminals via a first pair of capacitors (20a, 20b), a direct-current power supply connected between the first pair of terminals without interposition of the first pair of capacitors, and first and second switches (24a, 24b)connected in series to the direct-current power supply between the first pair of terminals, the first and second switches being disposed on opposite sides of the direct-current power supply, the second transmission device includes a second pair of terminals connected to the second electrical coupler via a second pair of signal lines, a reception unit connected to the second pair of terminals via a second pair of capacitors (43a, 43b), a load resistor connected between the second pair of terminals without interposition of the second pair of capacitors, and characterized in that the second transmission device includes at least one PIN diode (70a; 70b) connected in series to the load resistor between the second pair of terminals, and when the first and second switches are placed in an ON state, the direct-current power supply applies a forward bias to the PIN diode. a first transmission device disposed in a first vehicle including a first electrical coupler; and a second transmission device disposed in a second vehicle including a second electrical coupler electrically connected to the first electrical coupler, the second transmission device being capable of communicating with the first transmission device, wherein the first transmission device includes a first pair of terminals connected to the first electrical coupler via a first pair of signal lines, a transmission unit connected to the first pair of terminals via a first pair of capacitors (20a, 20b), a direct-current power supply connected between the first pair of terminals without interposition of the first pair of capacitors, and first and second switches (24a, 24b)connected in series to the direct-current power supply between the first pair of terminals, the first and second switches being disposed on opposite sides of the direct-current power supply, a first pair of terminals connected to the first electrical coupler via a first pair of signal lines, a transmission unit connected to the first pair of terminals via a first pair of capacitors (20a, 20b), a direct-current power supply connected between the first pair of terminals without interposition of the first pair of capacitors, and first and second switches (24a, 24b)connected in series to the direct-current power supply between the first pair of terminals, the first and second switches being disposed on opposite sides of the direct-current power supply, the second transmission device includes a second pair of terminals connected to the second electrical coupler via a second pair of signal lines, a reception unit connected to the second pair of terminals via a second pair of capacitors (43a, 43b), a load resistor connected between the second pair of terminals without interposition of the second pair of capacitors, and a second pair of terminals connected to the second electrical coupler via a second pair of signal lines, a reception unit connected to the second pair of terminals via a second pair of capacitors (43a, 43b), a load resistor connected between the second pair of terminals without interposition of the second pair of capacitors, and characterized in that the second transmission device includes at least one PIN diode (70a; 70b) connected in series to the load resistor between the second pair of terminals, and when the first and second switches are placed in an ON state, the direct-current power supply applies a forward bias to the PIN diode.