In this study, a novel reversible control method for particle dispersion/flocculation was developed. We found that the conformation of a polymer with solubility pressure dependencies on the side chain changed from random coil to fibril-like with ambient pressure. On the contrary, as we reported in our previous paper, the dispersion state of slurry changes with changes in the conformation of adsorbed polymers. Based on these results, we hypothesized that the dispersion state of the slurry can be reversibly controlled by ambient pressure. The sample used was aqueous slurry of alumina with a sodium salt of sulfonic acid copolymer with a relative molecular mass of 10000. The results showed that the dispersion state of slurry at atmospheric pressure was flocculated with a specific additive amount of polymer. On the contrary, after pressurization, the dispersion state was changed into a well dispersion state. Therefore, it was confirmed that the slurry was responsible for the pressure; thereby, a novel reversible control method for particle dispersion state was successfully developed.