A method and a device for detecting nucleic acid are disclosed, wherein impurities in a sample can be easily removed.. The method comprises injecting a sample containing an adsorption medium with nucleic adsorbed thereon into a chamber; washing the sample; heating the sample to denature the nucleic acid; cooling down; and detecting nucleic acid by using the biomolecule detection device. The device includes: a source and a drain region; a gate electrode layer; a chamber formed over the semiconductor substrate including the gate electrode layer; and a heating means, wherein a gate adsorption layer to which a nucleic acid is adsorbed is formed on the gate electrode layer. A single-stranded nucleic acid is adsorbed to the gate adsorption layer, and a channel is formed between the source and the drain region. The current in the channel provides a basis to detect a gene.