A p-channel metal oxide semiconductor field-effect transistor (PMOSFET)-type bio- sensor for detecting streptavidin-biotin protein complexes has been fabricated. Au, which has a chemical affinity with thiol, was used as the gate metal in order to form a self- assembled monolayer (SAM). A SAM was used to immobilize streptavidin. The hydroxyl group of SAM was bound with the amine group of streptavidin. Biotin was then injected into the solution to form streptavidin-biotin protein complexes in the solution. Streptavidin and biotin were bound by a high affinity (Ka~1015 Mol-1). Measurements were conducted in a phosphate buffer saline (PBS; pH 6.4, 20 M) solution and a Pt electrode was used as the reference electrode. The bindings of SAM, streptavidin, and biotin caused a variation in the drain current of the PMOSFET-type biosensor. To verify interactions among SAM, streptaividn, and biotin, a quartz crystal microbalance (QCM) measurement was per- formed.