1. An image forming apparatus to print print data on a recording paper, comprising: a concealing area specifying part that specifies a concealing area including a concealing character string in the print data; a character discriminating part that discriminates character attribute of the concealing character string; a see-through preventive image generation part that sets a see-through preventing area where shape and a size are same with the concealing area, generates multiple characters having the character attribute discriminated by the character discriminating part are at random, and generates a see-through preventive image respectively-arranging two or more generated characters at a random position in the see-through preventing area; and a printing part that generates print data for see-through preventing where the see-through preventive image is arranged at a position piled up by a same direction for the concealing area at the time of enclosure and prints on the recording paper.