Effects of reactant concentrations on the characteristics of reactive miscible viscous fingering in a Hele-Shaw cell at low finger-growth velocity were studied both experimentally and theoretically. The product distribution varies with the variations in the initial reactant concentrations and depends on the location of a reaction zone. When the reaction zone is located in the more-viscous-liquid region the product concentrates at the fingertips, but when it is located in the less-viscous-liquid region, the product spreads in a relatively broad area inside the fingers. This significant difference in the reaction pattern resulting from variations in the reactant concentrations is caused by the large difference of molecular diffusivity in the two liquids, that is, of viscosity which is one of the important factors for viscous fingering. These results are confirmed theoretically by one-dimensional diffusive-reactive analysis.