A method for detecting the position and the velocity by using an encoder mounted directly on a moving body wherein original signals of the encoder are pulse-width modulated by using a sampling carrier wave and the position is detected by measuring the width of the pulses thus obtained. An apparatus for implementing the method comprises a carrier wave generation circuit, a pulse width modulation circuit which pulse-width modulates the original signals coming from the encoder by using the carrier wave, a pulse width timer which measures the width of the pulses thus modulated, and a microcomputer connected to the pulse width timer.