1. A pupil detection apparatus comprising: an acquisition section that acquires an actual scale value of a peripheral area including a pupil of a user, the actual scale value being calculated from an image of the peripheral area; a first calculation section that calculates an actual scale prediction value of a pupil diameter of the user, based on a held actual scale prediction value of a previous pupil diameter of the user; a second calculation section that calculates a first number of pixels per unit length which is a target value of resolution, based on a resolution table and the calculated actual scale prediction value of the pupil diameter, the resolution table associating data of a plurality of pupil diameters with data of resolutions which correspond to the plurality of pupil diameters respectively; a normalization section that calculates a scale-up/scale-down factor based on the calculated first number of pixels per unit length, the acquired actual scale value of the peripheral area, and a number of pixels of an image of the peripheral area, and normalizes the image of the peripheral area based on the calculated scale-up/scale-down factor; and a detection section that detects an image of the pupil from the normalized image of the peripheral area.