1. A fingerprint reading method conducted by using a fingerprint reading system including a sweep type fingerprint sensor to read a series of partial fingerprint images representing a fingerprint from a finger which conducts slide operation, the fingerprint reading method comprising the steps of executing; an image acquisition process to successively acquire a partial fingerprint image by using the sweep type fingerprint sensor; a reconstruction process to make a partial fingerprint image acquired by said image acquisition process and an already acquired partial fingerprint image overlap each other in a corresponding portion, store a resultant image in a memory, and monitor to determine whether a definite region which becomes a region in which no new image change is caused by the storage has reached a predetermined size; a feature extraction process to generate feature information of the fingerprint by using an image in the definite region when the definite region reached at the predetermined size in said reconstruction process, and to release a memory portion corresponding to the definite region after the generation of feature information; and a feature construction process to successively record feature information generated in said feature extraction process in accordance with its generation order, and executing said reconstruction process on a subsequent partial fingerprint image by using a memory portion corresponding to the definite region, when the memory portion is released in said feature extraction process.