1. An image processing apparatus, comprising: a depth-adjustment-amount determination section configured to determine a depth adjustment amount of an input image; an evaluation function storage section configured to store an evaluation function, the evaluation function representing a relationship of a subjective depth amount of the image to a color-saturation component thereof, the subjective depth amount being a depth amount subjectively felt by a user; an input-value-adjustment-amount determination section configured to determine an input-value adjustment amount based on the evaluation function, the input-value adjustment amount representing a retina's response to the color-saturation component corresponding to the determined depth adjustment amount; an adjustment gain calculation section configured to calculate each spatial-frequency-component adjustment gain based on the determined input-value adjustment amount; and a gain superimposition section configured to superimpose each of the spatial-frequency-component adjustment gains on the color-saturation component of the input image, the spatial-frequency-component adjustment gains being calculated by the adjustment gain calculation section.