an alternative the operation electrode 4 and the opposing electrode 5 may be composed of platinum and any one of carbon gold and iridium the operation electrode 4 and the opposing electrode 5 may be formed by evaporation sputtering screen printing or plating when the operation electrode 4 and the opposing electrode 5 are formed by sputtering it is preferable that the titanium layer has a thickness in the range of 0 02 to 0 2 u03bcm and the platinum layer has a thickness in the range of 0 1 to 1 0 u03bcm the reference electrode 6 is comprised of a silver layer formed on the insulating substrate 8 and a silver chloride layer formed by chemically treating a surface of the silver layer a titanium layer and a platinum layer may be formed under the silver layer in order to enhance adhesion when the reference electrode 6 is formed by sputtering it is preferable that the silver layer has a thickness in the range of 0 1 to 1 0 u03bcm the silver chloride layer may be formed by treating a surface of the silver layer with iron chloride ii solution or chromium chloride ii solution or by electrolyzing the.