To unnecessitate a waste liquid treatment which is required in the case of disinfecting seed rice and also to surely disinfect seed rice.

Seed rice A is mixed with a powdery chemical B such as a disinfection agent, etc., by a mixer 10 together with a prescribed amount of water and moisture such as an impregnated liquid, etc. The chemical B is stuck on the surface of seed rice A in a humid powder coating state. The mixer 10 is provided with an opening part 14 formed in the cylinder part 13 of a rotary drum 11, a projected cover plate 19 internally provided in the rotary drum 11 so as to permit the inner side of the opening part 14 to communicate with the inside of the rotary drum 11 and a motor 31. When the rotary drum 11 is rotated in one direction, seed rice A, etc., is normally positioned in a part deeper than the opening part 14 and it is never discharged. When the rotary drum 11 is rotated in the opposite direction, seed rice A is supplied to an opening part 14 side in accordance with the rotation of the rotary drum 11 so as to be automatically discharged.