In an image-forming method, a ribbon-shaped thermal transfer sheet with a dye layer and a protective material layer arranged side-by-side in the longitudinal direction is moved, and thermal energy is applied from a thermal head while arranging a recording medium to face the dye layer to thermally transfer the dye layer onto the recording medium to form an image. Thermal energy is applied while arranging the image to face the protective material layer to thereby form a protective layer on the image by thermal transfer. A ribbon-shaped surface-property-modifying sheet including a surface-property-modifying region for modifying the surface of the protective layer is moved, and the protective layer is aligned with the surface-property-modifying region of the surface-property-modifying sheet. Heat and pressure are applied from the thermal head and the surface-property-modifying sheet is detached after cooling to modify the surface condition of the protective layer.