the period of the quadrant where the a phase signal has a positive value and the b phase signal has a positive value the area 1 represents the range of the quadrant where the a phase signal has a negative value and the b phase signal has a positive value the area 2 represents the period of the quadrant where the a phase signal has a negative value and the b phase signal has a negative value the area 3 represents the period of the quadrant where the a phase signal has a positive value and the b phase signal has a negative value referring to fig2 when the scale is sent at a nearly constant rate the quadrant determination circuit 38 outputs as an a d area signal the a phase signal and the b phase signal each in a nearly sine wave form as previously described the comparator 34 a issues an output signal ad and the comparator 34 b issues an output signal bd at the rising point of the output signal bd when the upper counter 41 is incremented so that the count value n of the upper area changes when the comparator 34 creates a carry signal or a digit taking up signal noises or impulses may occur near to the zero level in such a case the upper area 0 1 2 3 is created with the timing different from that in the quadrant inverted in the a d area at the point q 1 or q 2 thereafter when a counter request comes for example at.