Label-free biosensors, including conventional quartz-crystal-microbalance (QCM) biosensors, are seriously affected by nonspecific adsorption of contaminants involved in analyte solution, and it is exceptionally difficult to extract the sensor responses caused only by the targets. In this study, we reveal that this difficulty can be overcome with an ultrahigh-frequency, wireless QCM biosensor. The sensitivity of a QCM biosensor dramatically improves when the quartz resonator is thinned, which also makes the resonance frequency higher, causing high-speed surface movement. Contaminants weakly (nonspecifically) interact with the quartz surface, but they fail to follow the fast surface movement and cannot be detected as the loaded mass. The targets are, however, tightly captured by the receptor proteins immobilized on the surface, and they can move with the surface, contributing to the loaded mass and decreasing the resonant frequency. We have developed a MEMS QCM biosensor in which an AT-cut quartz resonator, 26 μm thick, is packaged without fixing, and we demonstrate this phenomenon by comparing the frequency changes of the fundamental (∼64 MHz) and ninth (∼576 MHz) modes. At ultrahigh-frequency operation with the ninth mode, the sensor response is independent of the amount of impurity proteins, and the binding affinity is unchanged. We then applied this method to the label-free and sandwich-free, direct detection of C-reactive protein (CRP) in serum and confirmed its applicability.