In a battery receiving chamber, the first convex part is formed in the internal side face of the battery compartment and the second convex part which corresponds to the first convex part is formed in the battery cover, the first and second convex part respectively having the inclined faces which gradually rise toward the rotational direction in which the battery cover is detached from the battery compartment, so that a battery receiving chamber having good usage convenience and highly reliable can be realized. Further, in a
hearing aid using the above, waterproof means is located so as to block the air vent formed in the battery cover, and a slot connecting the end part of battery cover and the air vent in the external side face of the battery cover is comprised, so that a
hearing aid which can supply practically sufficient volume of air into the case and has high waterproof performance can be realized. Further, the case of this
hearing aid is divided into two that the first half-case for receiving the earphone, and the second half-case for receiving the
microphone, and the joint part between the first and second-half cases is formed so as to substantially flat, in order to apply stress without bias to the waterproof member inserted into the joint part. Thereby, a hearing aid in which the waterproof performance can be further improved can be realized.