In recent years, the accidents that an accelerator pedal is mistakenly stepped happen frequently, most of the devices for preventing mistaken stepping on accelerators judge whether or not an accelerator pedals are mistakenly stepped according to the accelerator pedal change rate. When the accelerator pedals are mistakenly stepped, the rates vary from man to man, so that whether or not the accelerator pedals are mistakenly stepped is not likely to be judged by using a uniform change rate. When the mistaken stepping on accelerator pedals is studied, the researchers find that, in any running state of the vehicle, the opening-degree of the accelerator pedal exceeds 90% when the accelerator pedal is mistakenly stepped, so that judging whether or not the accelerator pedal is mistakenly stepped, according to the opening-degree of the accelerator pedal, is feasible. The invention provides a device which can judge whether or not the accelerator is mistakenly stepped according to the accelerator pedal opening-degree, and immediately sends an accelerator pedal initial signal (position signal of the accelerator pedal when the accelerator pedal is not stepped) to an Electronic Control Unit (ECU) when the accelerator pedal is mistakenly stepped by a driver. In addition, the device further has an accelerator pedal opening-degree-reminding function, when the driver steps on the accelerator pedal with a rush, the condition that the opening-degree of the accelerator pedal exceeds the opening-degree threshold value set for judging whether or not the accelerator pedal is mistakenly stepped is avoided.