Realizing an electronic phase competition in a material is a promising approach to induce a large electronic change by a small external field. One of typical materials showing the phase competition is the half-doped perovskite manganite, in which the ground state drastically changes between the antiferromagnetic-insulating (AFM-I) and ferromagnetic-metallic (FM-M) states depending on the slight difference in the band width. Near the phase boundary, the two phases coexist and compete with each other, giving rise to a gigantic field response like the colossal magnetoresistance. We show in this paper the observation of a self-organized anisotropic phase separation with use of a microwave impedance microscope for a thin film of Nd_0.5Sr_0.5MnO₃, which locates on the verge of the phase boundary. We also show an artificially tailored phase separation and its magnetic control realized in superlattices composed of Pr_0.5Ca_0.5MnO₃ (AFM-I) and La_0.5Sr_0.5MnO₃ (FM-M).