An absolute angle calculation apparatus, comprising: a rotary plate having patterns spatially modulated; a light emitting unit facing a rotary range of the patterns; a light receiving unit which receives a light beam obtained from the light emitting unit through the patterns and converts the light beam to photoelectrically-converted signal strings; and a calculation circuit which calculates an absolute angular position with respect to a reference angular position on the basis of a spatially modulated component contained in the photoelectrically-converted signal string outputted from the light receiving unit, the calculation circuit including: a photoelectrically-converted signal string storage which stores previous and current photoelectrically-converted signal strings outputted, for a purpose of interpolating the absolute angular position, from the light receiving unit at previous and current measurements within a set time shorter than an absolute angle calculation time required for a calculation of the absolute angular position; a correlation detection circuit which detects a correlation between the previous and current photoelectrically-converted signal strings stored in the photoelectrically-converted signal string storage to detect a shift amount therebetween; and an interpolation calculation circuit which calculates an inter-pattern absolute angular position equivalent value based on a shift amount obtained at the current measurement, and the calculation circuit calculating a current absolute angular position by calculating an interpolated angle based on the shift amount and the absolute angular position equivalent value, and then by adding a calculated interpolated angle obtained at the current measurement to an absolute angular position obtained at the previous measurement. a rotary plate having patterns spatially modulated; a light emitting unit facing a rotary range of the patterns; a light receiving unit which receives a light beam obtained from the light emitting unit through the patterns and converts the light beam to photoelectrically-converted signal strings; and a calculation circuit which calculates an absolute angular position with respect to a reference angular position on the basis of a spatially modulated component contained in the photoelectrically-converted signal string outputted from the light receiving unit, the calculation circuit including: a photoelectrically-converted signal string storage which stores previous and current photoelectrically-converted signal strings outputted, for a purpose of interpolating the absolute angular position, from the light receiving unit at previous and current measurements within a set time shorter than an absolute angle calculation time required for a calculation of the absolute angular position; a correlation detection circuit which detects a correlation between the previous and current photoelectrically-converted signal strings stored in the photoelectrically-converted signal string storage to detect a shift amount therebetween; and an interpolation calculation circuit which calculates an inter-pattern absolute angular position equivalent value based on a shift amount obtained at the current measurement, and a photoelectrically-converted signal string storage which stores previous and current photoelectrically-converted signal strings outputted, for a purpose of interpolating the absolute angular position, from the light receiving unit at previous and current measurements within a set time shorter than an absolute angle calculation time required for a calculation of the absolute angular position; a correlation detection circuit which detects a correlation between the previous and current photoelectrically-converted signal strings stored in the photoelectrically-converted signal string storage to detect a shift amount therebetween; and an interpolation calculation circuit which calculates an inter-pattern absolute angular position equivalent value based on a shift amount obtained at the current measurement, and the calculation circuit calculating a current absolute angular position by calculating an interpolated angle based on the shift amount and the absolute angular position equivalent value, and then by adding a calculated interpolated angle obtained at the current measurement to an absolute angular position obtained at the previous measurement.