PURPOSE: To enhance converting accuracy by eliminating the ripple of an output signal, by allowing a monomultivibrator to output a pulse signal having a constant pulse width and amplitude in response to a detection pulse signal of which the oscillation frequency changes corresponding to physical quantity.

CONSTITUTION: A monomultivibrator 11' inputs the detection pulse signal SG11 from an encoder for detecting the number of rotations of the drive shaft of a drive motor for running and the pulse cycle of the signal SG11 becomes short corresponding to the running speed of a feed vehicle, that is, in proportion to the number of rotations of the drive motor for running. The monomultivibrator 11' outputs a pulse signal SG12 having a constant pulse width and amplitude in response to the rising of the signal SG11 and said pulse signal SG12 is smoothed by a smoothing circuit 12' to be outputted to the next stage sample hold circuit 13'. The circuit 13' inputs and holds the output signal SG13 outputted from the smoothing circuit 12' for the predetermined time of said signal SG13 and outputs the held analogue value thereof as an angular speed detection signal SG14.