1. An image construction method used in an apparatus including a sensor mechanism to read a series of partial images representing a fingerprint from a finger which conducts a slide operation, the image construction method comprising the steps of: successively storing partial images read by the sensor mechanism, causing a displacement of a latest partial image in the successively stored partial images relative to an already acquired partial image with a pixel pitch of the sensor mechanism, and obtaining a difference quantity between the latest partial image and the already acquired partial image every displacement; conducting interpolation computation based on basis coordinates corresponding to a minimum difference quantity among the obtained difference quantities, first reference coordinates adjacent to the basis coordinates in a slide direction of a finger, and difference quantities obtained at the first reference coordinates and the basis coordinates, and thereby obtaining a first vector component indicating a movement quantity of the latest partial image in the slide direction as a value having a resolution higher than the pixel pitch; conducting interpolation computation based on the basis coordinates, second reference coordinates adjacent to the basis coordinates in a direction perpendicular to the slide direction, and difference quantities obtained at the second reference coordinates and the basis coordinates, and thereby obtaining a first candidate for a second vector component indicating a movement quantity of the latest partial image in the perpendicular direction as a value having a resolution higher than the pixel pitch; conducting interpolation computation based on the first reference coordinates, and third reference coordinates adjacent to the first reference coordinates in the perpendicular direction, and difference quantities obtained at the third reference coordinates and the first reference coordinates, and thereby obtaining a second candidate for the second vector component as a value having a resolution higher than the pixel pitch; conducting weighted average computation based on the first vector component on the first and second candidates for the second vector component, and thereby determining the second vector component; and disposing the latest partial image relative to the already acquired partial image on the basis of a movement quantity vector including the first and second vector components.