NOVELTY - An apparatus for constructing an immobilized DNA library comprises a reactor formed with a chemically modified substrate, a first heating and cooling unit for controlling temperature of the reactor, and a sample feeder unit for supplying a sample solution into the reactor, is new.    USE - The system with the required apparatus functionally connected is for constructing immobilized DNA library, gene amplification, analyzing amplified gene products and gene diagnosis, which is applicable in molecular biology, biochemistry, clinical chemistry, forensic science etc. e.g. in sequencing polymerase chain reaction (PCR)-amplified gene products to detect gene mutation and defects in gene diagnosis.    ADVANTAGE - Such apparatuses and the integrated system can be fully automated, with simplicity, efficiency and specificity in their operations e.g. PCR. Cassette-type immobilized DNA libraries are produced with ease of purification and drying in a short period of time, which can be used immediately or in a semi-permanent basis after storage. The setup in the reactor itself can be easily carried out.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for the following:    (1) a gene amplification apparatus similarly composed of the reactor formed with a chemically modified substrate, particularly substrate for immobilizing gene, first temperature control, and feeder;    (2) an apparatus for analyzing a gene amplification product in which the amplified gene product continuously supplied into it from the gene amplification apparatus is electrophoresed;    (3) a gene diagnosis apparatus consisted of the gene amplification apparatus, the apparatus for analyzing the gene amplification product, and a computer control unit; particularly in which data obtained from the supplied gene amplification product after electrophoresis are analyzed and processed by the computer;    (4) a method for controlling the apparatuses for constructing an immobilized DNA library, gene amplification and analyzing gene amplification, or for the gene diagnosis apparatus by inputting signals from the first temperature-measuring unit to the computer for comparison with the defined values for driving the second and third temperature control units;    (5) a method for performing electrophoresis of the supplied gene amplification product; and    (6) a method for analyzing and processing data from the electrophoresis.    DESCRIPTION OF DRAWING(S) - A simplified diagram of a gene diagnosis apparatus.