An abnormality detection system for a rotation angle sensor, comprising: a magnetic sensor that generates four electrical signals having different phases in accordance with rotation of a rotary shaft that is a sensing target; a temperature sensor that detects an ambient temperature of the magnetic sensor; and a control circuit that computes a rotation angle of the rotary shaft based on the four electrical signals generated by the magnetic sensor, wherein the four electrical signals include positive and negative two sine signals having a phase difference of 180° and positive and negative two cosine signals having a phase difference of 180°, the control circuit has a first detection function of detecting a fact that an abnormality has occurred in any one of the positive and negative two sine signals or in any one of the positive and negative two cosine signals based on a value of a sum of the positive and negative two sine signals and a value of a sum of the positive and negative two cosine signals, and a second detection function of detecting a fact that an abnormality has occurred in any one of the positive sine signal and the positive cosine signal or in any one of the negative sine signal and the negative cosine signal by comparing a first temperature computed based on a correlation between a temperature and an amplitude acquired from a value of a sum of squares of the positive sine signal and the positive cosine signal, a second temperature computed based on a correlation between a temperature and an amplitude acquired from a value of a sum of squares of the negative sine signal and the negative cosine signal, and a third temperature detected by the temperature sensor with each other, and the control circuit identifies which of the four electrical signals is abnormal by comparing results of detection obtained by the first detection function and the second detection function with each other. a magnetic sensor that generates four electrical signals having different phases in accordance with rotation of a rotary shaft that is a sensing target; a temperature sensor that detects an ambient temperature of the magnetic sensor; and a control circuit that computes a rotation angle of the rotary shaft based on the four electrical signals generated by the magnetic sensor, wherein the four electrical signals include positive and negative two sine signals having a phase difference of 180° and positive and negative two cosine signals having a phase difference of 180°, the control circuit has a first detection function of detecting a fact that an abnormality has occurred in any one of the positive and negative two sine signals or in any one of the positive and negative two cosine signals based on a value of a sum of the positive and negative two sine signals and a value of a sum of the positive and negative two cosine signals, and a second detection function of detecting a fact that an abnormality has occurred in any one of the positive sine signal and the positive cosine signal or in any one of the negative sine signal and the negative cosine signal by comparing a first temperature computed based on a correlation between a temperature and an amplitude acquired from a value of a sum of squares of the positive sine signal and the positive cosine signal, a second temperature computed based on a correlation between a temperature and an amplitude acquired from a value of a sum of squares of the negative sine signal and the negative cosine signal, and a third temperature detected by the temperature sensor with each other, and the control circuit identifies which of the four electrical signals is abnormal by comparing results of detection obtained by the first detection function and the second detection function with each other.