Apparatus and program for arctangent calculation

An arctangent calculation apparatus (1) includes a phase angle memory unit (2) in which a phase angle is stored in advance, the phase angle corresponding to a two-dimensional vector value in a range of 0° to 22.5°; a vector rotation unit (3) that performs a vector rotation operation on a two-dimensional vector value in a range of 0° to 360° and converts the two-dimensional vector value in a range of 0° to 360° into a two-dimensional vector value in a range of 0° to 22.5°; an arctangent calculation unit (4) that outputs, from the phase angle memory unit (2), a phase angle that corresponds to the two-dimensional vector value in a range of 0° to 22.5°; and a phase angle conversion unit (5) that converts the phase angle into a phase angle in a range of 0° to 360°.