A body motion detecting method calculates an acceleration-of-gravity vector from acceleration vectors, which are detection outputs of a multi-axial acceleration sensor. The method extracts a vertical component of acceleration, using the acceleration vectors from the acceleration sensor and the calculated acceleration-of-gravity vector. The method further detects peaks in the extracted vertical component of acceleration, and detects body motion in the vertical direction by analyzing the detected peaks. Related body motion detecting device and computer readable medium are also described.