1. An information restricting method comprising: receiving printing data; primarily storing the received printing data in a primary storage unit; secondarily storing specific information included in the printing data, in a secondary storage unit; detecting, from the printing data stored in the primary storage unit, the specific information stored in the secondary 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.