1. An image processing apparatus, comprising: a processing circuit configured to acquire three-dimensional histology information about tissue inside a subject and three-dimensional fluid information about fluid moving inside the subject, both of which are created based on a reflection wave of an ultrasonic wave transmitted to an inside of the subject, create three-dimensional particle information by removing an area in which the fluid is present in the three-dimensional fluid information at designated spatial intervals in order to replace a portion of flow data within the three-dimensional fluid information with a plurality of scattered, sparsely-distributed particles in three dimensions, each particle being represented by a graphical symbol having a predetermined shape, create a composite image based on the created three-dimensional particle information and the three-dimensional histology information, and control display of the composite image so that the composite image is displayed on a predetermined display.