PURPOSE: To decelerate a trailer simultaneously when a tractor is decelerated by a retarder device so as to improve vehicle stability by providing a pneumatic pressure type trailer brake device, cooperated with the retarder device of the tractor, in the tractor, in the case of the tractor for towing the trailer.

CONSTITUTION: A trailer brake device contains an electropneumatic conversion brake valve (23) associated with a retarder device parallelly arranged to a trailer brake valve (8) connected to an air tank in a tractor side air pressure circuit. In the electropneumatic conversion brake valve (23), compressed air of pressure corresponding to an operating angle of a retarder operating lever for actuating the retarder device is transmitted to a relay valve (11) of a trailer side air pressure circuit via a double check valve (9) to change an opening of the relay valve (11), and a pressure of compressed air, fed into a trailer brake chamber (13), is adjusted to control brake force of a trailer wheel.