A brake system in an electric drive dump truck. The electric drive dump truck has a generator 11 driven by an engine 10 and traveling motors 13L, 13R driven with electric power generated by the generator 11. Hydraulic brakes 20L, 20R, 21L, 20R are operated by brake pedal 18. An oil pressure sensor 22 detects hydraulic fluid pressure produced in accordance with the amount of depression of the brake pedal 18 and a control unit 14 controls the traveling motors 13L, 13R so as to operate them as generator-type retarders when the hydraulic fluid pressure P detected by the oil pressure sensor 22 is not smaller than a predetermined value P1.