In this paper, we show the relationship between pupil dilation and visualmanipulation tasks to measure the magnitude of individual habituation effect and task difficulty. Our findings show that pupil dilation can be used as a new physiological signal in the application of audience measurement, affective computing, affective communications, and user interface design. We built a pointer maze game where a subject moves a pointer from start to end positions on a straight pathway, and we observe the subject’s pupil size while changing the pathway width and performing the game repeatedly. Through the two experiments, we found the maximum pupil size increases during the game when the pathway narrows. The first experiment indicates the difficulty of the task (narrower pathway) is related to the larger pupil diameter. On the basis of these results, we built models relating to (1) pupil size and pathway width, (2) pupil size and duration, and (3) pathway width and duration. The second experiment indicates the pupil constriction is related to the habituation effect of the users. While a similar effect has already been reported, the magnitude of pupil dilation during our task was about ten times as high as that in other tasks, so our confidence in the model is high.