Micrometer-sized, hemispherical particles were successfully prepared as a result of the cleavage of Janus PMMA/PS composite particles by dispersion into acetone/water (9/1-10/0 v/v) media or a THF/water (8/2 v/v) medium. The spherical composite particles having a Janus structure were prepared by the slow evaporation of toluene from homogeneous PMMA/PS/toluene droplets dispersed in an aqueous medium in advance. It was clarified that the difference in affinity between PMMA and PS phases with respect to the media caused the cleavage of the composite particles. This method is expected to be a novel approach to the preparation of nonspherical polymer particles.