A rotation angle detector is provided with a frame-shaped yoke which includes a first wall and a second wall in parallel. A rotation axis of a detection subject extends inside of the frame-shaped yoke. A magnetic flux vector is uniformed in a direction orthogonal to the first wall, whereby even if a position of a Hall element deviates, the detected magnetic flux density is less varied. A robustness of the detector is enhanced. A distance between the Hall element and a second inner surface of the second wall is shorter than that between the Hall element and a first inner surface of the first wall. A length of the second inner surface is longer than a length of the first inner surface. At a vicinity of the second wall, the area in which the magnetic flux vector is uniformed can be enlarged. Thus, the robustness is further enhanced.