On December 16, 1997 about 700 people, mainly school children, were taken to hospitals after feeling sick while watching a TV animation of the Pokemon (i.e. Pocket Monsters) series. This is the Pokemon mass seizure incident in Japan. Many experts attributed the main cause to a 12 Hz red-blue flicker scene lasting about 5 s. It is well known that some physical attributes of the visual stimulus (e.g., flicker) can be detrimental to photosensitive people, or people vulnerable to visual stress. However, we think that cognitive factors also played a critical role in the incident. To investigate such factors, we asked normal adults to estimate the magnitude of visual stress induced by scenes selected from the Pokemon show and varied subjects’ knowledge of the story plot. We found that the estimated amount of visual stress depends on cognitive factors. Subjects who know the plot estimate the effect as significantly stronger than those who don’t know the plot.