To provide a mixer and seal structure which demonstrate an unconventionally epoch-making action effect.

In the mixer, fluid such as liquid or powder placed in a container 1 can be agitated by an agitation part 2 set in the container 1. In the agitation part 2, an agitation member 4 is protrudingly set on the peripheral surface of a rotary shaft 3 arranged to penetrate the container 1. A fluid leakage prevention mechanism S is set at a position where the rotary shaft 3 penetrates the container 1. A flange part 6 is set at a position which is a prescribed position of the peripheral surface of the rotary shaft 3 and arranged in the penetration part 5 of the container 1. A seal member 7 blocking the penetration part 5 is engaged with the rotary shaft 3, and a press body 8 which makes the seal member 7 contact with a rotating rotation surface 6a, the outside surface part of the flange part 6, in a pressing state is installed in the penetration part 5 to constitute the fluid leakage prevention mechanism S.