A nucleic acid analysis device which can determine a DNA sequence has a flowcell in which two or more DNA fragment clusters of two or more DNA fragments having identical nucleotide sequences are immobilized. At least a part of the flowcell is made of a transparent material. An irradiation unit irradiates a part in which the DNA fragment clusters are immobilized. The device has a lens for collecting fluorescence, and a light-detection element. A solution containing only dATP having a fluorescently modified phosphate terminal among four bases, a solution containing only dCTP having a fluorescently modified phosphate terminal among the four bases, a solution containing only dGTP having a fluorescently modified phosphate terminal among the four bases, a solution containing only dTTP having a fluorescently modified phosphate terminal among the four bases, and a buffer solution are sent sequentially to where the DNA fragment clusters are immobilized.