A silicon condenser microphone on an SOI (silicon on insulator) substrate using only one photo mask was fabricated. This microphone consists of a diaphragm with the thickness of 20 μm and the diameter of 2 mm, a SiO2 insulative spacer (4-μm-thick buried oxide), and a 450-μm-thick silicon back plate with the meshed structures having extremely small (60 μm) hexagonal shaped acoustic holes. The gap between the 20-μm-thick silicon diaphragm and the back plate is 4 μm, which is determined by the thickness of the buried oxide in the SOI wafer. This microphone was confirmed to function as a static pressure sensor. The SOI microphone was also connected to an amplifier circuit, and exposed to the sound pressure of 110 dB at the frequency of 1 kHz. The microphone clearly responded to the input sound, and the output ac voltage of approximately 40 V was detected.