In a method of measuring ion activity of a liquid sample comprising steps of spotting a reference liquid and the liquid sample on surfaces of ion-selective membranes, respectively, of at least a pair of ion-selective electrode sheets which is electrically insulated from each other, said ion-selective membranes being provided on the top of said ion-selective electrode sheets; and measuring a potential difference between the ion-selective electrodes under the conditions that both liquids are electrically connected to each other by a bridge, the improvement wherein:said each ion-selective electrode sheet is arranged upside down in such a manner that the ion-selective membrane is positioned on the lowest side; andsaid each liquid spotted from the upper side is temporarily conveyed downwardly to the lower level than the surface of the ion-selective membrane, and then conveyed upwardly to the surface of the ion-selective membrane so as to reach the surface of the ion-selective membrane. A device employable in the method is also disclosed.