To enable a parking brake of a trailer brake device to be automatically controlled in cooperation with an operation of a parking brake of a tractor even if a driver once does not ride off the tractor to operate a parking control valve of the trailer present outside a cabin when the parking brake of the trailer is operated.

A cut valve 27 is arranged between a parking control valve 25 and a pressure control valve 26 of a parking brake control system passage placed between a spring brake chamber 20 and an air tank 23 of a trailer brake device. This cut valve 27 is connected to an emergency brake line E at the trailer placed more upstream side of a relay emergency valve 21.