A motion determining apparatus for determining a motion of an input device including an acceleration sensor for detecting acceleration in each of at least two axial directions, the motion determining apparatus comprising: data obtaining means (30, S61, S64) for obtaining a time series of consecutive acceleration data (Da) which is output from the acceleration sensor; motion direction determination means (30, S51, S66, S71) for determining a motion direction of the input device with respect to a direction of an acceleration of gravity acting upon the input device, the motion direction determination means including circulation direction determination means for determining a circulation direction in which the time series of the consecutive acceleration data circulates around the origin of a two-dimensional coordinate system as a circulation axis, wherein the coordinate axes are defined based on components of the two axial directions of the acceleration data and the origin represents a value of the acceleration data in the state where no acceleration including the acceleration of gravity acts upon the acceleration sensor, the circulation direction being either clockwise or counterclockwise, and wherein the motion direction determination means determines a moving direction of the input device based on the circulation direction determined by the circulation direction determination means; and output means for outputting motion data (Dc, Dd, Dj, D1) including at least the motion direction determined by the motion direction determination means, wherein the acceleration sensor detects an acceleration including at least a centrifugal component generated when the input device is swung by a user to output the acceleration data; and the motion direction determination means determines the motion direction of the input device using acceleration data obtained in a time period in which an acceleration of the centrifugal component exceeds a threshold value, among the acceleration data obtained by the data obtaining means, the motion direction being either a leftward swing or a rightward swing. data obtaining means (30, S61, S64) for obtaining a time series of consecutive acceleration data (Da) which is output from the acceleration sensor; motion direction determination means (30, S51, S66, S71) for determining a motion direction of the input device with respect to a direction of an acceleration of gravity acting upon the input device, the motion direction determination means including circulation direction determination means for determining a circulation direction in which the time series of the consecutive acceleration data circulates around the origin of a two-dimensional coordinate system as a circulation axis, wherein the coordinate axes are defined based on components of the two axial directions of the acceleration data and the origin represents a value of the acceleration data in the state where no acceleration including the acceleration of gravity acts upon the acceleration sensor, the circulation direction being either clockwise or counterclockwise, and wherein the motion direction determination means determines a moving direction of the input device based on the circulation direction determined by the circulation direction determination means; and output means for outputting motion data (Dc, Dd, Dj, D1) including at least the motion direction determined by the motion direction determination means, wherein the acceleration sensor detects an acceleration including at least a centrifugal component generated when the input device is swung by a user to output the acceleration data; and the motion direction determination means determines the motion direction of the input device using acceleration data obtained in a time period in which an acceleration of the centrifugal component exceeds a threshold value, among the acceleration data obtained by the data obtaining means, the motion direction being either a leftward swing or a rightward swing.