Provided is a measurement method for measuring a size of a damaged portion in a coated metal material that includes a metal base and surface treatment film provided on the metal base, and reaching the metal base through the surface treatment film. The measurement method includes the steps of disposing a water-containing material to be in contact with the damaged portion and an electrode to be in contact with the water-containing material, and electrically connecting between the electrode and the metal base with an external circuit; applying, with the external circuit, a constant voltage between the electrode and the metal base, as a cathode and an anode, respectively, and measuring a current value flowing therebetween; and calculating a size of the damaged portion, based on the current value measured and a correlation between the current value and the size of the damaged portion, determined on an exploratory basis in advance.