A rotation angle detection device comprising: a plurality of sensors (71, 72, 73) used to detect a rotation angle of a rotary body (8; 9); a sampling unit that samples an output signal from each of the sensors at prescribed timings; a computation unit that computes the rotation angle of the rotary body (8; 9) with use of a plurality of the output signals from at least two sensors among the plurality of the sensors (71, 72, 73), the output signals of each at least two sensors being sampled at a prescribed number of different timings, the prescribed number being three or more; and a multipolar magnet that rotates in accordance with rotation of the rotary body (8; 9), and that has a plurality of magnetic poles, wherein a sensor group constituted of multiple sensors is selected from the plurality of the sensors (71, 72, 73), and the sensors arranged at respective ends of the sensor group respectively output sinusoidal signals having a phase difference smaller than 180° in electrical angle in accordance with rotation of the multipolar magnet; the computation unit is configured to compute the rotation angle of the rotary body (8; 9) with use of six output signals from two sensors that belong to the sensor group, the six output signals including three output signals from one of the two sensors and three output signals from the other one of the two sensors, and being sampled at three different timings, multiple expressions obtained by mathematizing the six output signals each include an amplitude of the corresponding output signal, the rotation angle of the rotary body (8; 9), and a piece of information regarding a magnetic pole width of each of the magnetic poles of the multipolar magnet as unknown quantities, the computation unit computes the rotation angle of the rotary body (8; 9) on an assumption that, among the unknown quantities included in the six expressions, the amplitudes of the three output signals from each one of the two sensors, the three output signals being sampled at different sampling timings, are equal to one another, and all the pieces of the information regarding the magnetic pole width included in the six expressions are the same. a plurality of sensors (71, 72, 73) used to detect a rotation angle of a rotary body (8; 9); a sampling unit that samples an output signal from each of the sensors at prescribed timings; a computation unit that computes the rotation angle of the rotary body (8; 9) with use of a plurality of the output signals from at least two sensors among the plurality of the sensors (71, 72, 73), the output signals of each at least two sensors being sampled at a prescribed number of different timings, the prescribed number being three or more; and a multipolar magnet that rotates in accordance with rotation of the rotary body (8; 9), and that has a plurality of magnetic poles, wherein a sensor group constituted of multiple sensors is selected from the plurality of the sensors (71, 72, 73), and the sensors arranged at respective ends of the sensor group respectively output sinusoidal signals having a phase difference smaller than 180° in electrical angle in accordance with rotation of the multipolar magnet; the computation unit is configured to compute the rotation angle of the rotary body (8; 9) with use of six output signals from two sensors that belong to the sensor group, the six output signals including three output signals from one of the two sensors and three output signals from the other one of the two sensors, and being sampled at three different timings, multiple expressions obtained by mathematizing the six output signals each include an amplitude of the corresponding output signal, the rotation angle of the rotary body (8; 9), and a piece of information regarding a magnetic pole width of each of the magnetic poles of the multipolar magnet as unknown quantities, the computation unit computes the rotation angle of the rotary body (8; 9) on an assumption that, among the unknown quantities included in the six expressions, the amplitudes of the three output signals from each one of the two sensors, the three output signals being sampled at different sampling timings, are equal to one another, and all the pieces of the information regarding the magnetic pole width included in the six expressions are the same.