There is provided a boundary microphone in which a microphone cover can be fixed to a base part without the use of an externally threaded screw for fixation. In the boundary microphone in which a condenser microphone unit 31 is housed in a microphone casing 1 including a flat base part 10 formed of a metallic magnetic material, the upper surface side of which is open, and a microphone cover 20 formed of a metallic magnetic material, which has many openings and covers the upper surface side of the base part 10, on either one of the base part 10 and the microphone cover 20, a permanent magnet 40 that magnetically attracts the other thereof is provided, and the microphone cover 20 is detachably fixed to the base part 10 by the permanent magnet 40.