A mixer capable of developing suitable stirring characteristics by forming a sufficient vertical circulation flow over the entire part of the inside of a tank, wherein an opening is formed in a stirring blade under the following conditions: (i) the ratio of the opening area of the opening in the upper half of the stirring blade to the overall area of the upper part of the stirring blade (the opening rate in the upper half of the stirring blade) is 45 to 80%, (ii) the ratio of the opening area of the opening in the lower half of the stirring blade to the overall area of the lower half of the stirring blade (the opening rate in the lower half of the stirring blade) is 18 to 50%, and (iii) the ratio of the opening rate in the lower half of the stirring blade to the opening rate in the upper half of the stirring blade is 31 to 71%.