1. A user state presumption system comprising: a category storage which stores state data indicating to which states defined in advance a plurality of users who accessed a server providing a service via a network belong; a transition model recorder which records, for a transition between the states defined in advance, transition condition data indicating a user behavior pattern serving as a condition for changing of a user from a certain state to another state; a log receiver which reads an access log containing data identifying the plurality of users who accessed the server, data indicating information to which the plurality of users referred and reference times at which the plurality of users referred to the information; a log abstracting part which detects, for each of the plurality of users, beginning and ending of a series of accesses made by the user on the basis of the access log read by the log receiver and abstracts, for a series of accesses made by the user, access history data containing data identifying information referred to by the user in the series of accesses; a behavior categorizing part which categorizes, on the basis of the information referred to by the user in the series of accesses indicated by the access history data abstracted by the log abstracting part, a behavior of the user in the series of accesses into one of a plurality of behavior patterns defined in advance; a category presuming part which presumes how a state of each of the plurality of users indicated by the state data stored in the category storage changes by comparing the behavior pattern of the user determined by the behavior categorizing part with the user behavior pattern serving as the condition for the transition between the states indicated by the transition condition data recorded in the transition model recorder and updates the state data on the basis of a result of the presumption; a transition probability storage which stores a transition probability of a user transition between the states defined in advance; a transition probability calculator which calculates, every predetermined period, the transition probability of the user transition between the states defined in advance by referring to the state data and stores the obtained transition probability in the transition probability storage; an intended behavior judging part which judges, on the basis of the access history data abstracted by the log abstracting part, whether the user has exhibited a specific intended behavior that is an aim of the service provided by the server in the series of accesses; an intended behavior probability recorder which records the proportion of users who have exhibited the intended behavior among users in individual states; and an intended behavior probability calculator which calculates, every predetermined period, the proportion of users who have exhibited the intended behavior among the plurality of users in the individual states on the basis of a result of the judgment obtained by the intended behavior judging part and the state data and records the obtained proportion in the intended behavior probability recorder.