The invention provides a measuring instrument and a refilling method, which are capable of suppressing a flow path from being clogged due to the precipitation of crystals from an internal solution or a calibration liquid even when kept in a waiting state. The measuring instrument 100 includes a measuring electrode 21 and a reference electrode 22 and on the basis of the potential difference generated therebetween, measures a characteristic value of a sample. In order to accomplish the above object, the measuring instrument 100 is adapted to be able to have two states, i.e., a measurement state of measuring the characteristic value of the sample and a waiting state of not performing measurement, and further include a refilling mechanism 8 that refills the calibration liquid or the internal solution used in the reference electrode. In addition, the refilling mechanism 8 is adapted to continuously or intermittently refill the calibration liquid or the internal solution in the waiting state.