A ball rotational direction detecting system includes a magnetic sensor that is fixed in a ball, an acceleration sensor, a bearing recording unit that records a moving bearing, a magnetic dip recording unit that records a magnetic dip α of geomagnetism, and a calculating unit. The calculating unit calculates the orientation of the rotation axis and the rotational direction of the ball with respect to a traveling direction F and a gravity direction G of the ball, based on a geomagnetism vector m, an acceleration vector a, the moving bearing, the magnetic dip α, and time variations in the geomagnetism vector m or the acceleration vector a.