A boundary microphone includes: a base made of metal; a cover that is made of metal and has a plurality of holes through which a sound wave is guided; a microphone unit that converts sound into an electric signal; and a microphone unit holder slidably provided on the base and holds the microphone unit. The microphone unit holder has a knob. The base has a hole through which the knob of the microphone unit holder penetrates the base. The knob of the microphone unit holder and the hole of the base are so provided that the microphone unit holder can be moved with the microphone unit by a movement of the knob within a range defined by the hole.