The absolute angle calculation apparatus includes a calculation circuit calculating an absolute angular position based on a spatially modulated component in a photoelectrically-converted signal string. The calculation circuit includes a photoelectrically-converted signal string storage storing previous and current photoelectrically-converted signal strings obtained, for interpolating the absolute angular position, within a set time shorter than an absolute angle calculation time required to calculate the absolute angular position; a correlation detection circuit detecting a correlation between the previous and current photoelectrically-converted signal strings in the photoelectrically-converted signal string storage and detecting a shift amount therebetween; and an interpolation calculation circuit interpolating an inter-pattern absolute angular position equivalent value based on the shift amount obtained by the current measurement. The calculation circuit calculates an interpolated angle based on the shift amount and the absolute angular position equivalent value, adds the calculated interpolated angle obtained at the current measurement to the absolute angular position obtained at the previous measurement to calculate a current absolute angular position.