Acceleration data which is output from an acceleration sensor is obtained. A rotation motion of an input device around a predetermined direction as a rotation axis is determined by comparing a start point in a two-dimensional coordinate system which is represented by the first acceleration data obtained in a predetermined period, and an end point in the two-dimensional coordinate system which is represented by the last acceleration data obtained in the predetermined period. Coordinate axes of the two-dimensional coordinate system are defined based on components of the two axial directions of the acceleration data, and an origin of the two-dimensional coordinate system represents a value of the acceleration data in the state where no acceleration including the acceleration of gravity acts upon the acceleration sensor. Motion data including at least the determined rotation motion is output.