Abstract: A pivotal support portion 2 protruding from a frame 1 is clamped between a pair of clamp ends 4 provided at the tip of an earpiece 3. A protrustons 7 formed in the inner surface of each of said pair of clamp ends 4 is fitted within a corresponding pivot hole 6 in each of the upper and bottom surface of said pivotal support portion 2. A lock mechanism 11 including protuberances 12 and recesses 13 is provided at a position where said clamp ends 4 are contacted with said frame 1. When said earpiece 3 is opened or enlarged to the maximum open position, said protuberances 12 become fitted within said respective recesses 13, so as to prevent said clamp ends 4 from separating in the outward direction.