Optical cavities using air-band states in Si photonic crystals (PhCs) are proposed as biosensors. Since the light in the air band of PhCs is preferably located at the lower-index cladding rather than the inside of higher-index Si, interactions are enhanced between the light and the target molecules in the cladding, being effective for the sensitive detections. Such an air-band PhC is used as an optical cavity, which is surrounded with another PhC as the photonic-bandgap mirrors. Simulations are performed for air-band resonators in one-dimensional (1D) PhC of arrayed holes in a Si channel waveguide, in order to analyze a figure of sensing, i.e., red shifts in the resonance wavelengths induced by the presence of target molecules. The S value, the amount of red shift caused by the unit change in the cladding index, can be as large as 200 nm/RIU, which is 2 times larger than those for ordinary 1D-PhC resonators. Fabricated devices successfully show the resonance peaks, whose wavelengths and quality factors agree well with theoretical ones. These results suggest that the air-band resonators are promising for high-performance biosensors. (C) 2014 The Japan Society of Applied Physics