PURPOSE: To prevent the lowering in the sensitivity of an electromagnetic acoustic converter of a small size used for a hearing aid, by forming an auxiliary magnetic circuit comprising a frame and a case for an outer circumference of a main magnetic circuit consisting of a magnet and a yoke.

CONSTITUTION: A yoke 11 consists of a base plate 12, upper and lower plates 13, 14 prolonged in parallel from the upper and lower ends of the base 12, projection pieces 15, 15' prolonged in parallel from both side surfaces of the upper plate 13 and projection pieces 16, 16' prolonged in parallel from both sides of the lower plate 14. Plate magnets 17, 17' are fixed to a surface opposed to the surfaces of the plates 13 and 14 of the yoke 11 opposing with each other. The magnets 17, 17' and the yoke 11 form a main magnetic circuit. The outer circumference is formed with an auxiliary magnetic circuit comprising a frame 18 and a case 11, allowing to prevent the lowering of the sensitivity of an electromagnetic acoustic converter without saturation of DC magnetic flux even with the thin yoke 11.