A method for measuring the size of one or more damaged portions formed in a coated metal material that includes a metal base (2, 3) and a surface treatment film provided on the metal base (2, 3), the one or more damaged portions reaching the metal base (2, 3) through the surface treatment film, the method comprising the steps of: removing adherents on the surface of the one or more damaged portions; and measuring the size of the one or more damaged portions after the step of removing the adherents, wherein the step of removing the adherents comprises the steps of: disposing a water-containing material to be in contact with one or two out of the one or more damaged portions and one or two electrodes to be in contact with the water-containing material, and electrically connecting, with an external circuit, between the electrode and the metal base (2, 3), or between the two electrodes; and supplying a current between the electrode and the metal base (2, 3), or between the two electrodes while alternately switching a direction of the current flowing through the external circuit, with the external circuit; and the step of measuring the size of the one or more damaged portions comprises the steps of: electrically connecting, with an external circuit, between the electrode and the metal base (2, 3); supplying a direct constant voltage between the electrode and the metal base (2, 3) serving as a cathode and an anode, respectively; detecting a current value flowing between the electrode and the metal base (2, 3); and calculating the size of the one or more damaged portions based on the measured current value and the correlation between the current value and the size of the one or more damaged portions determined on an exploratory basis in advance. removing adherents on the surface of the one or more damaged portions; and measuring the size of the one or more damaged portions after the step of removing the adherents, wherein the step of removing the adherents comprises the steps of: disposing a water-containing material to be in contact with one or two out of the one or more damaged portions and one or two electrodes to be in contact with the water-containing material, and electrically connecting, with an external circuit, between the electrode and the metal base (2, 3), or between the two electrodes; and supplying a current between the electrode and the metal base (2, 3), or between the two electrodes while alternately switching a direction of the current flowing through the external circuit, with the external circuit; and disposing a water-containing material to be in contact with one or two out of the one or more damaged portions and one or two electrodes to be in contact with the water-containing material, and electrically connecting, with an external circuit, between the electrode and the metal base (2, 3), or between the two electrodes; and supplying a current between the electrode and the metal base (2, 3), or between the two electrodes while alternately switching a direction of the current flowing through the external circuit, with the external circuit; and the step of measuring the size of the one or more damaged portions comprises the steps of: electrically connecting, with an external circuit, between the electrode and the metal base (2, 3); supplying a direct constant voltage between the electrode and the metal base (2, 3) serving as a cathode and an anode, respectively; detecting a current value flowing between the electrode and the metal base (2, 3); and calculating the size of the one or more damaged portions based on the measured current value and the correlation between the current value and the size of the one or more damaged portions determined on an exploratory basis in advance. electrically connecting, with an external circuit, between the electrode and the metal base (2, 3); supplying a direct constant voltage between the electrode and the metal base (2, 3) serving as a cathode and an anode, respectively; detecting a current value flowing between the electrode and the metal base (2, 3); and calculating the size of the one or more damaged portions based on the measured current value and the correlation between the current value and the size of the one or more damaged portions determined on an exploratory basis in advance.