A viewing distance estimation method includes: acquiring a first image captured by an image capturing device and including a face of a person who watches a target; detecting a size of an iris of the person from the first image; calculating a first value indicating a pixel number for the detected size of the iris; acquiring a resolution of the first image; calculating, based on the first value and a second value indicating a predetermined inherent dimension for the size of the iris, a third value indicating an actual dimension of one pixel; estimating a viewing distance corresponding to the acquired resolution and the calculated third value, based on relational information representing a relation among the resolution, the third value, and the viewing distance; and outputting estimative information including the estimated viewing distance.