Even when the size of a three-dimensional image is changed, the pop-out amount is automatically adjusted to one intended by the user. The pop-out amount is adjusted based on a conversion characteristic defining a relationship between the size and the pop-out amount of a three-dimensional image as the size of the three-dimensional image is changed, and therefore the pop-out amount of the three-dimensional image can be automatically adjusted to a given pop-out amount preferred by the user or intended by the user.