To provide a safety device to prevent a trailer from starting to travel under an imperfectly connected condition.

In a towing vehicle in which the hitch arm of a trailer is connected to the coupler of a towing vehicle detachably through a joint pin and connectors 19a and 19b are connected to each other so as to supply power from a towing vehicle side to the rear combination lamp 49 of the trailer, a pin detector switch 68 and a trailer detector switch 77 are installed to the connection part of the trailer and towing vehicle, and a connector connection detector circuit 101 for detecting the connection of the connectors 19a and 19b is provided. Also these connector connection detector circuit 101, pin detector switch 68, and the trailer detector switch 77 are connected to the ground side of a kill circuit of a CDI unit 100. Thus, in both conditions where the connectors are connected to each other but the trailer is not connected accurately or the connectors are not connected but the trailer is connected accurately, the kill circuit is ground and the output of the CDI unit 100 is ground so as to prevent the towing vehicle from starting to travel under the condition where the connection of the trailer is not perfect.