A breathing detection air bag is disclosed, which includes an air bag and a detector. The air bag includes a sponge body capable of being expanded and contracted with contraction and expansion of a pectoral or abdominal part of a person, a sponge cover enclosing the sponge body for preventing air contained in the pores of the sponge body from escaping from the pores, and a flange provided integrally with the edge of the sponge cover and an air guide tube for guiding an air stream caused by the expansion and contraction of the sponge body. The detector includes as oscillating film for causing a change in the electrostatic capacitance between it and a back electrode in response to a movement of air caused by expansion and contraction of the air bag, an electret capacitor microphone having a field-effect transistor for converting a change in the electrostatic capacitance into a change in voltage, and a pressure reduction film provided in front of the oscillating film for preventing mechanical saturation of the oscillating film when the extent of movement of air exceeds the distortion of the oscillating film.