1. An apparatus for forming a fingerprint image, comprising: a partial sensor including a first sensor and a second sensor that are configured to obtain partial images of a fingerprint of a finger moving over the partial sensor in a direction from the first sensor to the second sensor; a reference image storing unit configured to store a reference partial image obtained by the second sensor; and an image difference calculating unit configured to calculate a reference image difference between the reference partial image and a first partial image obtained by the first sensor, to calculate a current image difference between the reference partial image and a second partial image obtained by the second sensor, to calculate an amount of movement and/or a position of the finger based on a ratio of the current image difference to the reference image difference, and to store the second partial image and/or the first partial image as parts of the fingerprint image to be formed if the amount of movement and/or the position of the finger is equal to or greater than a predetermined value.