PROBLEM TO BE SOLVED: To provide a fluid treatment device and a fluid treatment method capable of optimizing influence on agitation performance by an opening.SOLUTION: A fluid treatment device includes a treatment section 1 disposed in fluid for which fluid treatment such as emulsification, dispersion or mixture is scheduled. The treatment section 1 includes an agitation chamber 7 and a rotating agitation blade 12 configured in the agitation chamber 7. The agitation chamber 7 has an opening 11 for introduction for introducing fluid therein and a screen 9 with a discharge port 8. The agitation blade 12 is rotated so as to introduce fluid from the opening 11 to the agitation chamber 7, and fluid treatment is performed by shearing fluid at a fine gap between the agitation blade 12 and the screen 9. Further, the fluid treatment device includes a flow passage adjustment section 20 for changing flow passage cross section area of the opening 11. The flow passage cross section area is changed so as to control particl
