A planetary mixer has stirring blades that undergo planetary motion within a tank in close proximity to an inner wall of the tank for stirring solid/liquid type treatment materials received by the tank. Each of the stirring blades includes a vertical side portion having two slope faces slanting toward the inner wall of the tank, an edge face connecting outward front ends of the slope faces, and an inner face connecting inward front ends of the slope faces. The outward front ends are disposed closer to the tank inner wall than are the inward front ends, with a distance between the inward front ends being greater than a distance between the outward front ends. The inner face is formed in the shape of an arc having a center located at an intersection of a line interconnecting the inward front ends and centerline running through a center of the edge face.