To actuate, control and adjust various kinds of equipment mounted on a tractor depending on the presence or absence of trailer connection with no provision of special switches.

An information-lamp-lighting control circuit 4 lights an information lamp 12 on when receiving an L-signal indicative of no ABS connection from a trailer-ABS-presence/absence decision circuit 2 as well as an H-signal indicative of trailer-stoplight connection from a trailer-stoplight-connection decision circuit 3. A trailer-presence/absence-signal generation circuit 5 outputs a trailer-presence/absence signal to various kinds of equipment mounted on a tractor when receiving an H-signal indicative of trailer-stoplight connection from the trailer-stoplight-connection decision circuit 3. This trailer-presence/ absence signal actuates, controls and adjusts the various kinds of equipment according to the presence or absence of trailer connection.