A rotation angle detecting method includes detecting a first rotation angle based on a first measured magnetic value received from a first hall element, detecting a first magnetic value received from a second hall element corresponding to the first detected rotation angle, adjusting a gain based on a first difference value between a second measured magnetic value received from the second hall element, in response to one of the magnetic pieces being rotated at the first rotation angle, and the first magnetic value, redetecting a second rotation angle by applying the gain to the first magnetic value and redetecting a second magnetic value received from the second hall element, corresponding to the second rotation angle, and outputting the second rotation angle as a confirmed rotation angle in response to a second difference value between the second magnetic value and the second measured magnetic value becoming less than a delta value.