An absolute value encoder for sensing the number of revolutions, wherein an A-phase/B-phase generating section (610) generates A-phase/B-phase one-period pulse signals the phases of which are 90 degrees different from each other within one revolution of the shaft on the basis of the light passing through a slit rotating together with the shaft. When an A-phase/B-phase status sensing section (640) senses a change of the pulse signals, a clock generating section (66) switches the clock frequency.