A digital microphone includes: a cavity resonator operatable in a micrometer, millimeter, or electromagnetic waveband, the cavity resonator having a metal wall including a conductive membrane 32 that vibrates in response to incident acoustic waves and converts the shift of the membrane 32 into a resonance frequency of the cavity resonator; an FM-signal generator that modulates the resonance frequency of the cavity resonator in response to the shift of the membrane 32 and outputs FM signals from the metal wall other than the membrane; and a ΔΣ-modulated-signal generator that generates ΔΣ-modulated signals from the FM signals. The FM-signal generator includes a slot 36, a micro-strip line 38, and a negative resistive element 40. The ΔΣ-modulated-signal generator includes an edge detector 42.