We have fabricated a metallic structure in an organic Mott insulator kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Cl. The periodic metallic domains of approximately 90 x 90 mu m(2) obtained by X-ray irradiation through a molybdenum mesh mask are visualized by scanning microregion infrared reflectance spectroscopy technique. No deterioration by irradiation is found in a range of nanometer to micrometer scales. We demonstrate that the present processing method is applicable for the fabrication of molecular electronic devices. (C) 2009 Elsevier Ltd. All rights reserved.