An agitator able to mix and disperse a variety of fluids of various fluid volumes with different viscosities ranging from low to high, and having excellent, cleanability and improved cooling efficiency is provided. The agitator comprises a rotating shaft 3 extending vertically inside an agitating vessel 2, and a flat paddle blade 4 mounted on the rotating shaft 3. The flat paddle blade has a bottom flat paddle blade portion 4a extending outwards from the bottom of the rotating shaft 3 and a rectangular upper flat paddle blade portion 4b extending upward from an upper part of each side end of the bottom flat paddle blade portion 4a. The dimensional ratio (b/a) of the blade diameter (b) of the bottom flat paddle blade portion 4a to the inner diameter (a) of the agitating vessel 2 is from 0.6 to 0.9, and the dimensional ratio (d/c) of the height (d) of an upper flat paddle blade portion 4b to the height (c) of the bottom flat paddle blade portion 4a is from 1 to 4. A passage 12 is formed inside the rotating shaft 3 and flat paddle blade 4 for passing a coolant medium therethrough.