An image output system includes a computer and a complex machine connected to the computer. A password generating section of the complex machine, in a case where a user ID transmitted from a transmitting section of the computer is stored in a HDD, generates a password which is different from the user ID, and the transmitting section transmits the password generated by the password generating section. A controller, in a case where the password received by a password receiving section and the password transmitted from the password transmitting section to the computer, controls an image forming section to output image data which is not combined with electronic watermark information by an electronic watermark information combining section.