A rotational velocity of an incoming ball can be estimated using a simple configuration that does not require the use of a specialized ball, and a trajectory of a ball applied with spin can also be predicted with good precision using a simple configuration.;A rotation estimation device 31 includes an acquisition section 33 and a rotation estimation section 34. The acquisition section 33 is configured to acquire state information including a measured position and a measured velocity of an incoming ball. The rotation estimation section 34 is configured to solve an aerodynamic model based on the measured position and the measured velocity of the ball at a first time point and on a provisional rotational velocity so as to perform processing to find a predicted position and a predicted velocity of the ball at a second time point different from the first time point for plural of the provisional rotational velocities, and so as to compute as an estimated rotational velocity either the provisional rotational velocity minimizing a difference between the predicted position at the second time point and the measured position at the second time point or a predicted rotational velocity at the second time point corresponding to the provisional rotational velocity minimizing the difference.
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