PURPOSE: To facilitate the shaft seal of a concrete mixer in low cost without lowering the quality of a product such as concrete by ejecting air from the gap between the rotary part and non-rotary part of a shaft seal device at predetermined wind velocity at the time of kneading and ejecting pressure water at predetermined flow velocity for a predetermined time after the completion of kneading.

CONSTITUTION: Before kneading is started, the electromagnetic type opening and closing valve 36 of an air supply pipe 34 is opened. Air is supplied into a pressure chamber 18 from an air supply pipe 34 through a fluid supply pipe 33 and a fluid supply passage 30 and ejected into a kneading tank from a ring- shaped gap 15 through a jet gap 17. After the discharge of a product is completed, a part of pressure water high in density is ejected from the ring-shaped gap 15 in place of air through the fluid supply pipe 30. By this constitution, cement or sand bonded to the ring-shaped gap 15 and not let fall by air is let fall.