How are the two stimuli presented at different distances compared with each other, and how is the degree of size-constancy by matching method determined? These are the problems of this paper. We consider "mental slide" or "mental sliding" as to the comparison of the two stimuli presented at different distances. Both the gradient of mental slide and the mental sliding distance determine the degree of size-constancy. We suggest a formula which predicts the degree of size-constancy by matching method and a size-distance relationship.