1. An image forming apparatus to print a printing data on a recording paper, comprising: a concealing area specifying part that specifies a concealing area including a concealing character string in the printing data; a character discriminating part that discriminates a character attribute of the concealing character string; a see-through preventive image generation part that sets a see-through preventing area where a shape and a size are same with a 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 the printing 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; wherein the character discriminating part, when the concealing character string is a mixture character string where characters having mixed a different character attribute, further discriminates a mixture ratio of each character attribute; the see-through preventive image generation part generates a character for each character attribute at random in the mixture ratio discriminated by the character discriminating part; and the character attribute includes a character system, a font, a character type including full width and half width, a character size, or a character color.