In this paper, effect of ionization of polymer on viscosity of polymer solution is investigated theoretically from the microscopic view point. Flory-Fox relation, which simulates dependence of viscosity of polymer solution on radius of gyration of polymer molecule, is extended to apply for ionized polymer solution. Size of ionized polymer molecules in solution was defined by the electrical balance between the repulsive force of fixed ions of polymer particles and the neutralization by free ions clustered as an ionic atmosphere around polymer ion. A simple model equation is introduced to express the effects of salt ion concentration in solvent and of polymer ion concentration on viscosity of polymer solution. Measured viscosity values of the ionic polymer solutions are well matched with those derived by the introduced model equation, and microscopic observation of polymer molecules in solution turned out to assist better understanding of polymer rheology.