Disclosed is an agitator including an agitation axis at the center of the agitator vessel, an agitator blade mounted on the agitation axis and consisting of a flat-plate-shaped blade disposed along the inner surface of the bottom wall of the agitator vessel and a grating-shaped blade continuous with the flat-plate-shaped blade, a baffle plate vertically extending for rotation around the agitator blade along the inner surface of the side wall of the agitator vessel, and drive equipment for rotating the baffle plate and the agitator blade independently from each other. When the agitator blade and the baffle plate are rotated at different speeds, circulating flow is formed in the vessel. The grating-shaped blade at the upper portion of the agitator blade shears a part of the circulating flow which descends along the radially inward agitation axis, thereby dividing that part of the flow into small parts. The small parts of the liquid are efficiently mixed together by the action of minute swirls generated behind the components of the grating-shaped blade.