A hearing aid which achieves a comfortable feeling to use while preventing generation of howling includes a microphone for converting an environmental sound to an electric signal, a hearing aid processor for hearing aid processing of an output signal of the microphone, an earphone for converting an output signal of the hearing aid processor to a sound signal, an external ear canal microphone converting a sound within an external ear canal to an electric signal, and an adaptive filter for comparing an output signal of the external ear canal microphone and the output signal of the hearing aid processor to automatically adjust the output signal of the hearing aid processor in such a manner that the output signal of the external ear canal microphone approaches the output signal of the hearing aid processor. The output signal of the hearing aid processor functions as an input signal and a desired signal of the adaptive filter, and the output signal of the external ear canal microphone functions as a response signal of the adaptive filter.