PURPOSE: To detect surely the rapid acceleration/deceleration state of a vehicle, by providing a simple variation detecting circuit such as a filter.

CONSTITUTION: When an accelerator pedal is stepped or released rapidly, high- frequency level variance components are generated in an accelerator signal S1 because the level of the accelerator signal S1 is changed rapidly in accordance with this stepping or releasing, and high-frequency level variance components are outputted as a rapid acceleration/deceleration signal K from a high-pass filter 15. An output signal Ks obtined by shaping the waveform of the signal K is taken out as a binary signal whose level becomes "H" in response to the rapid stepping of the accelerator pedal, and this signal Ks is inputted to an OR gate 17. An oscillation output U is applied to a speaker 19 to generate an alarm tone.