Abstract: An apparatus for measuring the quantities of chemical substances contained in a solution to be assayed by using a chemical-substance sensitive sensor consists of a potentiometric electrode which responds selectively to chemical substances, and a liquid-junction type reference electrode including a tube, a porous thread member housed in the front end opening of the tube and having a pore capacity of 0.2 cc/g or more and at least the surface of which is hydrophilic, a silver-silver chloride wire is provided and housed in the tube so that at one end of the wire is in contact with the thread member and the wire which extends along the tube. The wire is connected to a connector at the other end opening of the tube, an electrical insulating resin fills said tube, at least at the front end portion thereof, for fixing both the silver-silver chloride wire and the thread member by contact therewith and for stopping said tube. An internal solution is retained by suction in said porous thread member.