Having an outline font data readout unit read a correction subject pixel and surrounding pixels thereof, referring to the already stored pre-correction and post-correction brightness value pattern data that are related to the read correction subject pixel and surrounding pixels, deriving a post-correction brightness value pattern uniquely from among candidates for a post-correction brightness value pattern of an appropriate value as post-correction brightness value, determining the optimal post-correction brightness value from the uniquely derived post-correction brightness value pattern and outputting correction data based on the determined optimal post-correction brightness value, thereby changing a brightness value of a pixel within a character image giving a look of unnaturalness automatically to a brightness value not giving a look thereof.