In this study the relation between the transport number of counterion and the concentration of Donnan salt was investigated to elucidate the decrease of the transport number in higher concentrations of electrolyte solution equilibrated with the membrane. In the experiment three different types of membranes were examined, and the membrane potential and membrane conductance were measured. In addition, the ionic concentration within the membrane was determined to estimate the transport number due to Donnan salt, tD. The tD was given as a function of Donnan salt concentration, and the mean mobilities of ions which exist as Donnan salts within the membrane were estimated. The values of ŪD also depended upon the valences of counterions and types of membranes. As a result, it was pointed out that the transport numbers of Donnan salts in cases of higher concentration ranges of outer electrolyte solutions were apparently close to those in the aqueous solution, and the decrease of transport numbers evaluated from membrane potential were attributed to the low values of tD.