An LED 103 and a photodiode 104 of a crank angle detector confront one another across a gap G', through which a plate 102 rotates. The LED and photodiode communicate through respective housing windows 111 when a slit 110 in the plate passes between them. The windows and the slit are sector-shaped, with concentric inner and outer edges, and radially oriented side edges. These configuration steepen the leading and trailing edges of the photodiode output, thus enhancing the detection accuracy.