A device for measuring ionic activity is composed of: a block of insulating material having a hollow space therein, a solution-receiving surface area in which a pair of openings for receiving a sample solution and a reference solution separately are provided, a plurality of solution-supplying surface areas in each of which a pair of openings for supplying outside the sample solution and the reference solution separately are provided; a bridge member fixed on the solution-receiving surface area; a guide member placed in the hollow space which assists to transmit separately the sample solution and the reference member in the hollow space; and two or more ion selective electrodes each of which is placed on each solution-supplying surface area.