In a rotation angle detection apparatus, a signal detected by a magnetoresistive element is digitalized before being subjected to subtraction of the optimum correction parameter therefrom in a subtractor, the optimum correction parameter being stored in advance in a memory. Note that during the initial transmission at the time of activation, the data size “n” of a detection target obtained by a CPU from a control device is transferred to the memory, so that the optimum correction parameter for the data size of the detection target is selected. With the above, the detection unit can relatively readily cope with a request for enlargement or the like of a through hole size made by a machine side, and detection accuracy is not deteriorated and an error in absolute position processing is reduced when the curvature of the detection target is changed.