A rotation angle detection device comprising: a multipolar magnet that rotates in accordance with rotation of a rotary body, and that has a plurality of magnetic poles; three magnetic sensors that respectively output sinusoidal signals having a predetermined phase difference in accordance with rotation of the multipolar magnet; a sampling unit that samples an output signal from each of the magnetic sensors at prescribed timings; a first computation unit that computes a rotation angle of the rotary body based on the output signals from two magnetic sensors among the three magnetic sensors, the output signals being sampled at three sampling timings, when a condition that both the two magnetic sensors among the three magnetic sensors sense one and the same magnetic pole for three consecutive sampling periods is satisfied, and computes information regarding a magnetic pole width of the magnetic pole sensed by the two magnetic sensors and information regarding amplitudes of the output signals from the two magnetic sensors and stores the information regarding the magnetic pole width and the information regarding the amplitudes in association with the magnetic pole sensed by the two magnetic sensors, at all times or when the output signals sampled at the three sampling timings satisfy a prescribed requirement; and a second computation unit that computes the rotation angle of the rotary body based on the information stored by the first computation unit and the output signals from two magnetic sensors among the three magnetic sensors, the two magnetic sensors including one of the three magnetic sensors, which senses the magnetic pole associated with the stored information regarding the magnetic pole width, when the condition is not satisfied. a multipolar magnet that rotates in accordance with rotation of a rotary body, and that has a plurality of magnetic poles; three magnetic sensors that respectively output sinusoidal signals having a predetermined phase difference in accordance with rotation of the multipolar magnet; a sampling unit that samples an output signal from each of the magnetic sensors at prescribed timings; a first computation unit that computes a rotation angle of the rotary body based on the output signals from two magnetic sensors among the three magnetic sensors, the output signals being sampled at three sampling timings, when a condition that both the two magnetic sensors among the three magnetic sensors sense one and the same magnetic pole for three consecutive sampling periods is satisfied, and computes information regarding a magnetic pole width of the magnetic pole sensed by the two magnetic sensors and information regarding amplitudes of the output signals from the two magnetic sensors and stores the information regarding the magnetic pole width and the information regarding the amplitudes in association with the magnetic pole sensed by the two magnetic sensors, at all times or when the output signals sampled at the three sampling timings satisfy a prescribed requirement; and a second computation unit that computes the rotation angle of the rotary body based on the information stored by the first computation unit and the output signals from two magnetic sensors among the three magnetic sensors, the two magnetic sensors including one of the three magnetic sensors, which senses the magnetic pole associated with the stored information regarding the magnetic pole width, when the condition is not satisfied.