A sensor apparatus comprising: a first sensor unit; a second sensor unit; and a control unit connected to the first sensor unit and the second sensor unit, wherein the first sensor unit includes a first Hall sensor and a second Hall sensor, the first Hall sensor outputs two first output values S1, C1, having different phases, the second Hall sensor outputs two second output values S2, C2, having different phases, the second sensor unit outputs a third position detection value (Θ3) of a rotor, and characterized by the fact that the first sensing unit and the second sensing unit are disposed along or adjacent an external circumference surface of a sensing magnet of the rotor, wherein the control unit is configured to determine the following differences, in sequence, between - one of the two first output values S1, C1, with one of the two second output values S2, C2, - the two first output values S1, C1, with each other, - the two second output values S2, C2, with each other, - a first position detection value (Θ1) of the rotor based on the first output values S1, C1, with a second position detection value (Θ2) of the rotor based on the second output values S2, C2, - at least one of the first position detection value (Θ1) of the rotor and the second position detection value (Θ2) of the rotor with the third position detection value (Θ3) of the rotor output from the second sensor unit, wherein the control unit is further configured to determine that the rotor and the sensors are abnormal if a first one in the sequence of said differences of output values or position detection values exceeds, respectively, a first threshold or a second threshold. a first sensor unit; a second sensor unit; and a control unit connected to the first sensor unit and the second sensor unit, wherein the first sensor unit includes a first Hall sensor and a second Hall sensor, the first Hall sensor outputs two first output values S1, C1, having different phases, the second Hall sensor outputs two second output values S2, C2, having different phases, the second sensor unit outputs a third position detection value (Θ3) of a rotor, and characterized by the fact that the first sensing unit and the second sensing unit are disposed along or adjacent an external circumference surface of a sensing magnet of the rotor, wherein the control unit is configured to determine the following differences, in sequence, between - one of the two first output values S1, C1, with one of the two second output values S2, C2, - the two first output values S1, C1, with each other, - the two second output values S2, C2, with each other, - a first position detection value (Θ1) of the rotor based on the first output values S1, C1, with a second position detection value (Θ2) of the rotor based on the second output values S2, C2, - at least one of the first position detection value (Θ1) of the rotor and the second position detection value (Θ2) of the rotor with the third position detection value (Θ3) of the rotor output from the second sensor unit, wherein - one of the two first output values S1, C1, with one of the two second output values S2, C2, - the two first output values S1, C1, with each other, - the two second output values S2, C2, with each other, - a first position detection value (Θ1) of the rotor based on the first output values S1, C1, with a second position detection value (Θ2) of the rotor based on the second output values S2, C2, - at least one of the first position detection value (Θ1) of the rotor and the second position detection value (Θ2) of the rotor with the third position detection value (Θ3) of the rotor output from the second sensor unit, wherein the control unit is further configured to determine that the rotor and the sensors are abnormal if a first one in the sequence of said differences of output values or position detection values exceeds, respectively, a first threshold or a second threshold.