A ribbon microphone includes a ribbon microphone unit; an acoustic box for mounting a rear acoustic terminal of the ribbon microphone unit; a detective microphone mounted in the acoustic box, the detective microphone detecting sound waves identical to sound waves guided to the rear acoustic terminal of the ribbon microphone unit; a speaker comprising a diaphragm, the speaker being assembled in the acoustic box and varying the pressure in the acoustic box in response to the driven diaphragm; and a drive unit for driving the speaker so as to cancel a variation in pressure in the acoustic box in response to signals detected by the detective microphone, the variation being caused by sound waves guided to the rear acoustic terminal. This configuration extracts an omnidirectional component without an acoustic tube to achieve a small high-sensitivity unidirectional ribbon microphone and a unidirectional converter for the ribbon microphone.