In an abnormality detection system for a rotation angle sensor, a fact that an abnormality has occurred in one of sine signals (S1, S2) or in one of cosine signals (S3, S4) is detected based on a sum of the electrical signals (S1, S2), and a sum of the electrical signals (S3, S4) (first detection result). A fact that an abnormality has occurred in one of the positive sine and cosine signals (S1, S3) or in one of the negative sine and cosine signals (S2, S4) by comparing a first temperature computed using a correlation between an amplitude and a temperature and based on the electrical signals (S1, S3), a second temperature computed using a correlation between the amplitude and a temperature and based on the electrical signals (S2, S4), and a third temperature detected by a temperature sensor with each other (second detection result). A microcomputer identifies which of the electrical signals is abnormal by comparing the first and second detection results with each other.