1. An image forming apparatus having a function of superimposing a pattern on a target output image when conducting an image outputting process, the pattern superimposable based on parameters set for one or more document properties, the image forming apparatus comprising: a memory to store an administrator-set parameter settable for each one of the document properties of the superimposed pattern, and a user-set parameter settable for each one of the document properties of the superimposed pattern; a mode determination unit to identify a pattern superimposing mode, to be used for superimposing the pattern on the target output image, from a plurality of pattern superimposing modes settable for the image forming apparatus, each one of the pattern superimposing modes configurable by using the administrator-set parameter alone, the user-set parameter alone, or a combination of the administrator-set parameter and the user-set parameter; a pattern information obtaining unit to obtain the administrator-set parameter or the user-set parameter for each one of the document properties to be used for superimposing the pattern on the target output image based on the identified pattern superimposing mode and by referencing a property-mode correlation table correlating each one of the document properties and the plurality of pattern superimposing modes; an image drawing information generator to generate image drawing information for the target output image superimposed with the pattern based on the parameter obtained by the pattern information obtaining unit; and an image outputting unit that processes the image drawing information and conducts the image outputting process.