A method of measuring an ion-concentration characterized by a sample absorbed in an easily absorbing solution material and the absorbing material is brought into contact with a measuring portion to measure an ion-concentration 10/90 in the sample so that a highly accurate measurement can be achieved even though a quantity of the sample is small. In addition, the desired measurement can be carried out without using any instruments for sampling and needing any complicated pretreatments. A sheet for use in the calibration of an ion-concentration 10/90 measuring device characterized by a calibrating buffer solution absorbed in a chemically stabilized, hygroscopic, water-absorbing sheet eluting no ions and then the water-absorbing sheet is dried.