1. An image forming system, comprising: an image forming apparatus including a storage unit storing an image-forming job log containing log images indicating a history of images output in jobs performed by the image forming apparatus; and a management server communicably connected with the image forming apparatus via a network, the management server having a storage unit storing the image-forming job log as transmitted to the management server from the image forming apparatus via the network, and the management server having a control unit constituted by control circuitry, wherein the control circuitry is programmed to operate as a target-log-image extracting unit performing optical character recognition on the image-forming job log to extract text information from the log images in the image-forming job log so as to extract, on the basis of the extracted log-image text information, a document-including target log image likely to have been generated by use for a specific purpose of the image forming apparatus, and a relevant-log-image extracting unit extracting, from comparison-target log images in the image-forming job log other than the target log image, a log image similar to the target log image, as a relevant log image, based on a specific feature of the target log image extracted by the target-log-image extracting unit, the specific feature being at least document ruled-line arrangement as one of document format items according to which the target-log-image document is formatted, and if a comparison-target log image includes a document formatted with a ruled-line arrangement similar to at least a specific extent to the ruled-line arrangement of the target-log-image document, the relevant-log-image extracting unit extracts the comparison-target log image as the relevant log image.