1. An information restricting method comprising: receiving printing data; storing the received printing data in a primary storage unit; referring to a secondary storage unit, in which specific information was stored beforehand, to detect the specific information from the printing data stored in the primary storage unit; rendering an image forming unit forming an image based on the printing data stored in the primary storage unit; and restricting the image formation of the detected specific information on a prescribed medium by replacing the specific information with prescribed data.