A quartz crystal oscillation unit was modified by immobilizing single-stranded DNA (ssDNA) on its surface and incorporated into a quartz crystal microbalance (QCM) system as a biosensor for the purpose of detecting specific fungus. The ssDNA having base sequence specific to 28S rDNA D2 domain of Omphalotus guepiniformis, which is a toadstool that often causes a false appetite resulting in poisoning, was immobilized on a gold electrode of the quartz crystal unit, and used as a probe for detection of complementary target DNA. The detection of target DNA was carried out by monitoring the decrease in oscillation frequency due to mass increase on the surface of the quartz crystal unit, which accompanied the hybridization of the probe and target DNAs to form double strands. In advance, the DNA hybridization was examined by the QCM method with a synthesized oligonucleotide (25 bases) having same sequence as that specific to O. guepiniformis 28S rDNA D2 domain, and the hybridization efficiency was estimated to be 97-101%. Subsequently, DNA was extracted from a minute piece (0.1 g) of O. guepiniformis fruit body, and the 28S rDNA D2 fragment was amplified by polymerase chain reaction (PCR). It was confirmed that fragment (308 bases) of the PCR product was detected by the QCM measurement which showed a marked decrease in oscillation frequency. In addition, the 28S rDNA D2 fragment of O. guepiniformis was detected for a mixture with those of Lentinus edodes and Pleurotus ostreatus. This result suggests that the DNA-modified quartz crystal unit can be applied to the detection of a specific toadstool included in leftover foods or vomited matter.