Microcystin is a bacterial hepatotoxin generated from the cyanobacteria causing waterblooms. A DNA aptamer binding to microcystin was screened by the in vitro selection method of twelve rounds in order to obtain a sorbent suitable for a microcystin detection system. The obtained DNA aptamer showed specific binding to microcystin. The direct detection range using an immobilized aptamer and a surface plasmon resonance biosensor was 50 mug/ml to 1000 mug/ml.