We explore a flavor structure of quarks in the standard model under the assumption that flavor symmetries exist in a theory beyond the standard model, and chase after their properties, using a bottom-up approach. We reacknowledge that a flavorsymmetric part of Yukawa coupling matrix can be realized by a rank-one matrix and a democratic-type one occupies a special position, based on Dirac's naturalness. * E-mail: haru@azusa.shinshu-u.ac.jp 1 The flavor structure of quarks and leptons has been studied intensively, based on various flavor symmetries [6,7,8,9,10,11,12,13].