A rotational-angle detection device, comprising: a magnet part provided to rotate integrally with a rotary shaft, and magnetized so that a position of an area magnetized as an N pole and a position of an area magnetized as an S pole are alternately switched as the rotary shaft rotates; a sensor part comprising a plurality of magnetic sensors (Ss1, Sc1, Ss2, Sc2) arranged so as to oppose the magnet part, each of the plurality of magnetic sensors outputting a periodic signal that periodically changes by a magnetic action of the magnet part as the rotary shaft rotates; rotational-angle calculation means for calculating a rotational angle of the rotary shaft based on the periodic signal output by the each of the plurality of magnetic sensors (Ss1, Sc1, Ss2, Sc2) of the sensor part; shaft misalignment detection means for detecting occurrence of a shaft misalignment that is a misalignment in a relative position in a radial direction of the rotary shaft between the magnet part and the sensor part; and shaft misalignment information output means for outputting shaft misalignment information representing the occurrence of the shaft misalignment, characterized in that the sensor part comprises four magnetic sensors (Ss1, Sc1, Ss2, Sc2) for outputting periodic signals that change in a sine waveform as the rotary shaft rotates so that phases of the periodic signals are shifted from one another by π/2; and the shaft misalignment detection means determines that the shaft misalignment has occurred when a deviation between a first rotational angle calculated based on output signals of two of the four magnetic sensors (Ss1, Sc1, Ss2, Sc2) shifted from each other in the phases by π/2 and a second rotational angle calculated based on output signals of other two of the four magnetic sensors (Ss1, Sc1, Ss2, Sc2) shifted from each other in the phases by π/2 is larger than a threshold. a magnet part provided to rotate integrally with a rotary shaft, and magnetized so that a position of an area magnetized as an N pole and a position of an area magnetized as an S pole are alternately switched as the rotary shaft rotates; a sensor part comprising a plurality of magnetic sensors (Ss1, Sc1, Ss2, Sc2) arranged so as to oppose the magnet part, each of the plurality of magnetic sensors outputting a periodic signal that periodically changes by a magnetic action of the magnet part as the rotary shaft rotates; rotational-angle calculation means for calculating a rotational angle of the rotary shaft based on the periodic signal output by the each of the plurality of magnetic sensors (Ss1, Sc1, Ss2, Sc2) of the sensor part; shaft misalignment detection means for detecting occurrence of a shaft misalignment that is a misalignment in a relative position in a radial direction of the rotary shaft between the magnet part and the sensor part; and shaft misalignment information output means for outputting shaft misalignment information representing the occurrence of the shaft misalignment, characterized in that the sensor part comprises four magnetic sensors (Ss1, Sc1, Ss2, Sc2) for outputting periodic signals that change in a sine waveform as the rotary shaft rotates so that phases of the periodic signals are shifted from one another by π/2; and the shaft misalignment detection means determines that the shaft misalignment has occurred when a deviation between a first rotational angle calculated based on output signals of two of the four magnetic sensors (Ss1, Sc1, Ss2, Sc2) shifted from each other in the phases by π/2 and a second rotational angle calculated based on output signals of other two of the four magnetic sensors (Ss1, Sc1, Ss2, Sc2) shifted from each other in the phases by π/2 is larger than a threshold.