We have developed a very small, high-sensitivity silicon condenser microphone, in which a backchamber with concave lateral sides and a specific diaphragm were used as improvements. The concave sides increase the volume of the backchamber and thus solve the trade-off between chip downsizing and sensitivity. The novel backchamber was fabricated using a combination of isotropic and anisotropic etching. The diaphragm is supported only at the four corners to yield good deformation. The chip is 1.2 mm×1.3 mm×0.4 mm. The sensitivity is -41 dBV (8.8 mV/Pa) at 1 kHz and the frequency response is flat between 100 Hz and 10 kHz.