PURPOSE: To agitate and mix up a fluid to quickly make its composition, viscosity and temperature uniform while it is heated or cooled from the fluid passage side, by a method wherein a plate element having a twisted construction and through-holes is installed in a fluid passage, and the through-holes that do not contact with the fluid are brought in contact with thermal energy.

CONSTITUTION: A plate element 2 having a twisted construction is installed in a fluid passage such as a pipe, etc., to agitate and mix up a fluid, and two or more through-holes that do not contact with fluid are drilled in the plate element 2. Liquid or vapor heating or cooling medium is brought in contact with the through-holes to heat or cool the fluid from inside the fluid passage via the element 2 while the fluid passes through a mixer. In addition to heating or cooling from outside the fluid passage by a conventional heat source the fluid is heated or cooled via the element 2 from inside the fluid passage while it is agitated and mixed by the element 2 having a twisted construction, so that its composition, viscosity and temperature become uniform in a short period of time.