A detecting device obtains a first body motion signal from a body motion sensor while a person walks with a predetermined stride, the body motion sensor being attached to a body of the person and configured to detect a body motion of the person, obtains a reference-walk-waveform from a memory, the reference-walk-waveform being generated from a second body motion signal obtained from the body motion sensor while the person walks with the predetermined stride with the body motion sensor attached to an initially attached position, compares the obtained reference-walk-waveform with a waveform of the obtained first body motion signal to determine whether an attached position of the body motion sensor is shifted from the initially attached position, and outputs a determination result.