1. A contents' relationship visualizing apparatus comprising: at least one memory storing instructions; and at least one processor programmed via the instructions to: acquire a usage history comprising, for each one of a plurality of items of content, a user set comprising a plurality of elements each corresponding to a user who has utilized the one item of content; compare the plurality of elements in each user set to identify a set of elements common to each user set; calculate a similarity between the user sets by comparing the elements common to each user set to at least one of a total number of elements in at least one user set, a minimum number of elements contained in each user set, and a distance between the user sets; generate information representing a network, the network indicating one or more relationships between the plurality of items of content along a time-line comprising a plurality of usage time periods, the one or more relationships being based at least in part on the calculated similarity between the user sets.