A paid operation queue Q1 and a trial operation queue Q2 are provided within an output operation execution system. Each request for execution of an operation job from a client device is entered into either the paid operation queue Q1 or the trial operation queue Q2. An operation job is selected and executed to generate image output data. A paid operation job is selected in precedence to a trial operation job. The image output data thus obtained is temporarily stored into a storage device. If the executed operation job is a paid operation job, the image output data is transferred to the client device without undesired modification, and if the executed operation job is a trial operation job, the image output data is added with a trial-job mark and then is transferred to the client device.