A microphone unit has a diaphragm vibrating in response to sound waves; a unit casing accommodating the diaphragm; and a connecting path connecting a front acoustic terminal and a rear acoustic terminal. The unit casing has a small-diameter internal periphery defining the front acoustic terminal; a large-diameter internal periphery accommodating built-in components including the diaphragm; and a shoulder provided between the small-diameter internal periphery and the large-diameter internal periphery and positioning the built-in components. The unit casing has a groove in an axial direction provided in the large-diameter internal periphery and a groove in a radial direction provided in the shoulder and being in communication with the groove in the axial direction. The groove in the axial direction and the groove in the radial direction configure the connecting path.