A rotation angle detection apparatus, comprising: a magnet that rotates as a rotor rotates, and that has multiple magnetic poles arranged in a circumferential direction of the magnet; a first magnetic sensor that outputs, as the magnet rotates, a sinusoidal signal on which a third harmonic component is superimposed; a second magnetic sensor that outputs, as the magnet rotates, a sinusoidal signal on which a third harmonic component is superimposed and of which a phase is different from a phase of the sinusoidal signal output from the first magnetic sensor; a third harmonic component removing unit that corrects the sinusoidal signals output from the first magnetic sensor and the second magnetic sensor so as to remove the third harmonic components from the sinusoidal signals; and a rotation angle calculating unit that calculates a rotation angle of the rotor based on the sinusoidal signals corrected by the third harmonic component removing unit, wherein the third harmonic component removing unit comprises: a unit that calculates a post-correction signal V1a of the sinusoidal signal output from the first magnetic sensor according to an equation a; and a unit that calculates a post-correction signal V2a of the sinusoidal signal output from the second magnetic sensor according to an equation b, V ⁢ 1 ⁢ a = V ⁢ 1 ⋅ 1 1 + 4 ⁢ K 1 - K ⋅ 1 - V ⁢ 1 ⁢ b 2 V ⁢ 2 ⁢ a = V ⁢ 2 ⋅ 1 1 + 4 ⁢ K 1 - K ⋅ 1 - V ⁢ 2 ⁢ b 2 wherein K is a third harmonic component gain that is larger than zero but smaller than 1 (0 < K < 1), V1 is the sinusoidal signal output from the first magnetic sensor, V1a is the post-correction signal that is obtained by correcting the output signal V1 using the third harmonic component removing unit, V1b is a pre-correction signal that is the output signal V1 before correction, V2 is the sinusoidal signal output from the second magnetic sensor, V2a is the post-correction signal that is obtained by correcting the output signal V2 using the third harmonic component removing unit, and V2b is a pre-correction signal that is the output signal V2 before correction. a magnet that rotates as a rotor rotates, and that has multiple magnetic poles arranged in a circumferential direction of the magnet; a first magnetic sensor that outputs, as the magnet rotates, a sinusoidal signal on which a third harmonic component is superimposed; a second magnetic sensor that outputs, as the magnet rotates, a sinusoidal signal on which a third harmonic component is superimposed and of which a phase is different from a phase of the sinusoidal signal output from the first magnetic sensor; a third harmonic component removing unit that corrects the sinusoidal signals output from the first magnetic sensor and the second magnetic sensor so as to remove the third harmonic components from the sinusoidal signals; and a rotation angle calculating unit that calculates a rotation angle of the rotor based on the sinusoidal signals corrected by the third harmonic component removing unit, wherein the third harmonic component removing unit comprises: a unit that calculates a post-correction signal V1a of the sinusoidal signal output from the first magnetic sensor according to an equation a; and a unit that calculates a post-correction signal V2a of the sinusoidal signal output from the second magnetic sensor according to an equation b, V ⁢ 1 ⁢ a = V ⁢ 1 ⋅ 1 1 + 4 ⁢ K 1 - K ⋅ 1 - V ⁢ 1 ⁢ b 2 V ⁢ 2 ⁢ a = V ⁢ 2 ⋅ 1 1 + 4 ⁢ K 1 - K ⋅ 1 - V ⁢ 2 ⁢ b 2 wherein K is a third harmonic component gain that is larger than zero but smaller than 1 (0 < K < 1), V1 is the sinusoidal signal output from the first magnetic sensor, V1a is the post-correction signal that is obtained by correcting the output signal V1 using the third harmonic component removing unit, V1b is a pre-correction signal that is the output signal V1 before correction, V2 is the sinusoidal signal output from the second magnetic sensor, V2a is the post-correction signal that is obtained by correcting the output signal V2 using the third harmonic component removing unit, and V2b is a pre-correction signal that is the output signal V2 before correction. a unit that calculates a post-correction signal V1a of the sinusoidal signal output from the first magnetic sensor according to an equation a; and a unit that calculates a post-correction signal V2a of the sinusoidal signal output from the second magnetic sensor according to an equation b, V ⁢ 1 ⁢ a = V ⁢ 1 ⋅ 1 1 + 4 ⁢ K 1 - K ⋅ 1 - V ⁢ 1 ⁢ b 2 V ⁢ 2 ⁢ a = V ⁢ 2 ⋅ 1 1 + 4 ⁢ K 1 - K ⋅ 1 - V ⁢ 2 ⁢ b 2 wherein K is a third harmonic component gain that is larger than zero but smaller than 1 (0 < K < 1), V1 is the sinusoidal signal output from the first magnetic sensor, V1a is the post-correction signal that is obtained by correcting the output signal V1 using the third harmonic component removing unit, V1b is a pre-correction signal that is the output signal V1 before correction, V2 is the sinusoidal signal output from the second magnetic sensor, V2a is the post-correction signal that is obtained by correcting the output signal V2 using the third harmonic component removing unit, and V2b is a pre-correction signal that is the output signal V2 before correction.