To solve a problem of inability to cope with a case when an open door is slowly opened so as not to make a sound.

Since an alarm apparatus is provided with a sound pressure sensor 3 for detecting a low frequency sound due to an air flow generated when an open door D is opened/closed, a storage means 6 for previously storing data of wavelength patterns of low frequency sounds which may be generated in a plurality of sorts of open doors D, a CPU 5 for comparing an electric signal from the sound pressure sensor 3 with stored data extracted from the storage means 6, and when both the data coincide with each other, outputting a detection signal, and an alarm means 7 to be driven by an output signal from the CPU 5, an air flow which may be surely generated when the open door D is opened/closed is targeted and thus it is possible to cope with a case even when the open door D is slowly opened.