An example information processing device calculates an attitude of an input unit having a magnetic sensor. The information processing device repeatedly obtains detected magnetic vectors detected by the magnetic sensor. The information processing device repeatedly estimates a center position of a spherical body having a curved surface which is estimated based on end point positions of the detected magnetic vectors. The attitude of the input unit is calculated based on a direction vector representing a direction from the center position to the end point position of the detected magnetic vector. The information processing device calculates the attitude while relatively decreasing an influence of a newly-obtained detected magnetic vector as the end point position of the newly-obtained detected magnetic vector is farther away from the end point positions of the detected magnetic vectors used for the estimation of the center position.