A mixer for preparation of gypsum slurry, which has a circular housing defining a mixing area for mixing and kneading of the gypsum slurry, a rotary disc positioned in the housing and rotated in a predetermined rotational direction, a rotary driving shaft integrally connected with the rotary disc, a scraper positioned in the mixing area, and a slurry discharge port provided on the housing for feeding the gypsum slurry of the mixing area onto a production line: wherein said rotary driving shaft extends through an upper or lower plate (21, 22) of said housing to be connected with said rotary disc; wherein an inner end portion of said scraper is positioned in a center region of said rotary disc, and an outer end portion of the scraper is positioned in a peripheral zone of the disc; and wherein (i) and/or (ii): (i): the scraper is bent or curved backward in a rotational direction of the disc between said inner and outer end portions; (ii): said slurry discharge port is positioned on an annular wall of said housing and is provided with a fluid passage dividing member (47, 49) which divides an opening of the port into a plurality of narrow openings (48, 48') so as to increase fluid resistance on the gypsum slurry flowing out of said mixing area through said opening of the port, and the mixer further comprises an annular basal part positioned in said mixing area in concentricity with a rotational center of said rotary disc, wherein the annular basal part is rotated integrally with said disc in said housing, and wherein an inner end portion of said scraper is fixed to the annular basal part. wherein said rotary driving shaft extends through an upper or lower plate (21, 22) of said housing to be connected with said rotary disc; wherein an inner end portion of said scraper is positioned in a center region of said rotary disc, and an outer end portion of the scraper is positioned in a peripheral zone of the disc; and wherein (i) and/or (ii): (i): the scraper is bent or curved backward in a rotational direction of the disc between said inner and outer end portions; (ii): said slurry discharge port is positioned on an annular wall of said housing and is provided with a fluid passage dividing member (47, 49) which divides an opening of the port into a plurality of narrow openings (48, 48') so as to increase fluid resistance on the gypsum slurry flowing out of said mixing area through said opening of the port, and the mixer further comprises an annular basal part positioned in said mixing area in concentricity with a rotational center of said rotary disc, wherein the annular basal part is rotated integrally with said disc in said housing, and wherein an inner end portion of said scraper is fixed to the annular basal part. (i): the scraper is bent or curved backward in a rotational direction of the disc between said inner and outer end portions; (ii): said slurry discharge port is positioned on an annular wall of said housing and is provided with a fluid passage dividing member (47, 49) which divides an opening of the port into a plurality of narrow openings (48, 48') so as to increase fluid resistance on the gypsum slurry flowing out of said mixing area through said opening of the port, and the mixer further comprises an annular basal part positioned in said mixing area in concentricity with a rotational center of said rotary disc, wherein the annular basal part is rotated integrally with said disc in said housing, and wherein an inner end portion of said scraper is fixed to the annular basal part.