To provide a measuring method which enables a highly accurate measurement of concentrations of specific components in subjects of measurement even when the concentration measuring contact of a concentration measuring instrument is contaminated with drinks or the like.The concentration measuring instrument includes:a main body that has a concentration measuring contact, a light source and a photodetector; andjudging means that calculates the difference between a non-contact-measured value, a value, measured by the photodetector while keeping a subject of measurement out of contact with the concentration measuring contact, of the quantity of light that is emitted and entered by the light source into the concentration measuring contact and returned to the concentration measuring contact and a predetermined reference value obtained in advance by making measurement while keeping the concentration measuring contact clean and judges whether the calculation of the concentration of the specific component in the subject of measurement is effective or not by comparing the calculated difference with a predetermined threshold.