Purpose: purpose to understand the influence of a goalkeeper's Müller-Lyer posture illusion on the offense's judgement on what direction to throw the ball. Methods: In order to achieve the study objective, ten handball players with more than ten year experience and other ten beginners with experience less than a year participated in the experiment. Every subject performed the task of perception/judgement by watching the video of a goalkeeper on the screen varied by the Müller-Lyer posture illusion, the task of judging the size, the task of judging location of the ball, and the task of throwing the ball. For the experiment, a beam projector, screen, video of goalkeeper's Müller-Lyer posture illusion and mouse motion tracing equipment were used. The group and condition of showing stimulation were the independent variables. Results: In case of the experienced players, first, the wing out posture of the goalkeeper looked bigger in the task of perception/judgement while there were no differences of the results between the task of judging location desired to be thrown and the location of the thrown ball in accordance with the posture. In case of the beginner players, the task of perception/judgement, task of judging location desired to be thrown and the location of the thrown ball had all different results in accordance with the illusion posture of the goalkeeper. In other words, they tried to throw it farther in the wing out posture while throwing it closer in the wing in posture when the goalkeeper looked smaller.