In general, a particle size breakup apparatus is disclosed herein. More particularly, the present invention proposes a rotor/stator type mixer that implements the particle size breakup apparatus and includes a stator having a plurality of openings formed thereon and a rotor disposed on the inside of the stator and spaced away form the stator with a specific gap, wherein the mixer can improve the shearing stress applied to a fluid to be processed and exhibit the higher performance and wherein the mixer can change and/or adjust the shearing stress applied to the fluid to be processed and change and/or adjust the rate at which the fluid to be processed is allowed to flow in accordance with the changed and/or adjusted shearing stress. The rotor, which is disposed inwardly of the stator having the plurality of openings formed thereon so that it can be spaced away from the stator with the specific gap, has a rotor peripheral wall that faces opposite the inside of the stator peripheral wall and is disposed inwardly radially of the peripheral wall of the stator having the plurality of openings formed thereon so that it can be spaced away from the stator with the specific gap. In addition, the rotor has a plurality of openings formed thereon.
