The purpose of the present invention is to provide an auxiliary braking device of a vehicle which obtains braking force corresponding to the amount of pressing of an accelerator pedal even during an operation of the auxiliary braking device and prevents frequent repetition of the acceleration and deceleration, to attain a fuel saving operation. Therefore, in the present invention, a vehicle electronic control unit (10) and other electronic control units (20, 30) are provided, the vehicle electronic control unit (10) and the other electronic control units are connected with each other by an in-vehicle communication system (L123), and an auxiliary brake switch position detection device (5S) that detects a switch position of an auxiliary brake switch (5) and an accelerator pedal pressing amount measurement device (4S) that measures the amount of pressing of the accelerator pedal (4) are provided.