To provide a rotation angle detector capable of determining a trouble by only output signals.

In this rotation angle detector, the first and second targets 3b, 3c mutually prime in their numbers are provided respectively in rotors 12b, 12c rotated coaxially, and a plurality of respective detection means C, D, E, F arranged respectively opposedly to the targets 3b, 3c to output detection signals different each other in phases in response to respective positions of the targets 3b, 3c, according to rotation of the rotors 12b, 12c, detect rotation angles of the rotors 12b, 12c, based on the detection signals output respectively. The rotation angle detector is provided with a map 15 for storing values taken respectively by the plurality of detection signals output from the plurality of detection means (C, D or E, F), and for storing a prescribed range including the values, as a combination correlated to a mutual relation of the plurality of detection signals, and a means for determining the propriety of including the combination of the detection signals output respectively from the plurality of detection means (C, D or E, F), in the map 15.