An image processing apparatus that is connectable to a plurality of information processing devices (20a, 20b, 20c) via a network, the image processing apparatus comprising: an operation receiver configured to receive an operation input from a user; an authentication controller configured to authenticate a user of an information processing device (20a, 20b, 20c) that connects to the image processing apparatus and a user who operates the operation receiver; an image processor configured to perform image processing to add a digital watermark containing address information of the image processing apparatus to a first image to be processed by the image processing apparatus to thereby generate a second image; a communication controller configured to receive a taking-out request to take out the digital watermark contained in the second image from the information processing device (20a, 20b, 20c); and a storage controller configured to store, in a storage unit, a log representing an operation of the image processing apparatus that is performed according to an instruction of the user who is authenticated by the authentication controller, wherein when the communication controller receives the taking-out request, the image processor performs image processing to take out the digital watermark contained in the second image to thereby generate the first image, the communication controller transmits the first image that is generated by the image processor to the information processing device (20a, 20b, 20c), and the storage controller stores, in the storage unit, the log containing identification information that identifies the user of the information processing device (20a, 20b, 20c) from which the taking-out request is transmitted. an operation receiver configured to receive an operation input from a user; an authentication controller configured to authenticate a user of an information processing device (20a, 20b, 20c) that connects to the image processing apparatus and a user who operates the operation receiver; an image processor configured to perform image processing to add a digital watermark containing address information of the image processing apparatus to a first image to be processed by the image processing apparatus to thereby generate a second image; a communication controller configured to receive a taking-out request to take out the digital watermark contained in the second image from the information processing device (20a, 20b, 20c); and a storage controller configured to store, in a storage unit, a log representing an operation of the image processing apparatus that is performed according to an instruction of the user who is authenticated by the authentication controller, wherein when the communication controller receives the taking-out request, the image processor performs image processing to take out the digital watermark contained in the second image to thereby generate the first image, the communication controller transmits the first image that is generated by the image processor to the information processing device (20a, 20b, 20c), and the storage controller stores, in the storage unit, the log containing identification information that identifies the user of the information processing device (20a, 20b, 20c) from which the taking-out request is transmitted.