6. An image log management method comprising: (a) storing correspondence relationship information between an identifier of an input document subject to image forming processing, an identifier of one or more output documents resulting from the image forming processing of the input document, and an identifier of image log data of the one or more output documents; (b) storing the image log data of the one or more output documents; (c) inputting document disposal information including an identifier of a disposal document that has been disposed of; (d) changing, based on the document disposal information, a status of a document corresponding to the disposal document to disposal-complete; and (e) selecting, based on the status of the document corresponding to the document disposal information and based on one or more identifiers of input or output documents stored in the correspondence relationship information, image log data requiring deletion and executing deletion processing thereon, wherein when all of the one or more output documents, processed by the image forming processing with the disposal document as the input document, have their respective status changed to disposal-complete, the image log data of the one or more output documents is deleted.
6. An image log management method comprising: (a) storing correspondence relationship information between an identifier of an input document subject to image forming processing, an identifier of one or more output documents resulting from the image forming processing of the input document, and an identifier of image log data of the one or more output documents; (b) storing the image log data of the one or more output documents; (c) inputting document disposal information including an identifier of a disposal document that has been disposed of; (d) changing, based on the document disposal information, a status of a document corresponding to the disposal document to disposal-complete; and (e) selecting, based on the status of the document corresponding to the document disposal information and based on one or more identifiers of input or output documents stored in the correspondence relationship information, image log data requiring deletion and executing deletion processing thereon, wherein when all of the one or more output documents, processed by the image forming processing with the disposal document as the input document, have their respective status changed to disposal-complete, the image log data of the one or more output documents is deleted. 7. The image log management method of claim 6 , wherein when all of the one or more output documents, processed by the image forming processing with the disposal document as the input document, have their respective status changed to disposal-complete, and all other output documents with a descendant relationship to the one or more output documents have their respective status changed to disposal-complete, the image log data of the one or more output documents and the image log data of the other documents is deleted.