1. A reliability evaluation system for an information system, comprising: an information inputting unit configured to input information necessary for reliability evaluation of the information system provided with an instrument, containing at least information concerning constituents indicating both software for executing a function of the instrument and hardware constituting the instrument, or concerning failures corresponding to failure modes of a constituent group formed of the constituents, system configuration information indicating information concerning a configuration of the information system employing the instrument and any of the constituents and the constituent group, and a standard value of system availability indicating availability of the entire information system; an information analyzing unit having a processor programmed to analyze information on the failures of each of the failure modes of any of the constituents and the constituent group, and thereby obtaining analysis information thereon; a fault-tree producing unit configured to produce a fault tree indicating hierarchical logical relationships from a top event indicating a failure mode of the information system to basic events indicating the failure modes of any of the constituents and the constituent group on the basis of the system configuration information; an unavailability calculating unit configured to calculate unavailability of any of the constituents and the constituent group corresponding to the failure modes of any of the constituents and the constituent group on the basis of the analysis information a system availability calculating unit configured to calculate, on the basis of the fault tree and the unavailability corresponding to the basic events, availability corresponding to the top event as system availability in a case where it is supposed that the calculated unavailability is unavailability corresponding to the basic events equivalent to the failure modes of any of the constituents and the constituent group; an availability determining unit configured to determine whether or not the calculated system availability meets the standard value of the system availability; a basic-event extracting unit configured to extract the basic event related to an increase in the system availability by use of a relationship between variation in the unavailability corresponding to the basic events and variation in the unavailability corresponding to the top event, in a case where it is determined that the calculated system availability does not meet the standard value; and a first resetting unit configured to reset new analysis information corresponding to the extracted basic event in a case where it is possible to reduce the unavailability corresponding to the extracted basic event, and to reset new system configuration information and information concerning a failure corresponding to a new failure mode in a case where it is not possible to reduce the unavailability corresponding to the extracted basic event.