An auxiliary braking device of a vehicle, comprising: a vehicle electronic control unit, an engine electronic control unit, and a retarder electronic control unit, which units are connected by an in-vehicle communication system, an auxiliary brake switch position detection device for detecting a switch position of an auxiliary brake switch of an auxiliary braking device, the auxiliary braking force of which is changed in response to switch positions of the auxiliary brake switch, and an accelerator pedal pressing amount measurement device for measuring an amount of pressing of an accelerator pedal, wherein the vehicle electronic control unit is provided with a storage device storing characteristics of the amount of pressing the accelerator pedal and the auxiliary braking force, wherein the characteristics of the amount of pressing the accelerator pedal and the auxiliary braking force stored in said storage device are different at each switch position of the auxiliary brake switch, wherein the vehicle electronic control unit has a function for determining the auxiliary braking force corresponding to the amount of pressing of the accelerator pedal, and in correspondence with the characteristics stored in said storage device, such that if the accelerator pedal is pressed when the auxiliary braking device is operated by the auxiliary brake switch, the auxiliary braking force is determined based on accelerator opening information from the accelerator pedal pressing amount measuring device and information from the auxiliary brake switch position detection device in correspondence with said characteristics stored in said storage device, and wherein the vehicle electronic control unit has a function for transmitting the determined auxiliary braking force to the engine electronic control unit and the retarder electronic control unit. a vehicle electronic control unit, an engine electronic control unit, and a retarder electronic control unit, which units are connected by an in-vehicle communication system, an auxiliary brake switch position detection device for detecting a switch position of an auxiliary brake switch of an auxiliary braking device, the auxiliary braking force of which is changed in response to switch positions of the auxiliary brake switch, and an accelerator pedal pressing amount measurement device for measuring an amount of pressing of an accelerator pedal, wherein the vehicle electronic control unit is provided with a storage device storing characteristics of the amount of pressing the accelerator pedal and the auxiliary braking force, wherein the characteristics of the amount of pressing the accelerator pedal and the auxiliary braking force stored in said storage device are different at each switch position of the auxiliary brake switch, wherein the vehicle electronic control unit has a function for determining the auxiliary braking force corresponding to the amount of pressing of the accelerator pedal, and in correspondence with the characteristics stored in said storage device, such that if the accelerator pedal is pressed when the auxiliary braking device is operated by the auxiliary brake switch, the auxiliary braking force is determined based on accelerator opening information from the accelerator pedal pressing amount measuring device and information from the auxiliary brake switch position detection device in correspondence with said characteristics stored in said storage device, and wherein the vehicle electronic control unit has a function for transmitting the determined auxiliary braking force to the engine electronic control unit and the retarder electronic control unit.