Data on inspection, diagnosis, repair and operation histories are entered in a plant-maintenance apparatus for equipment that constitute a plant to be maintained. Failure-event tree breakdown is performed on failures to be expected for the equipment. Unreliability is predicted of at least one starting item in the failure-event tree breakdown. Unreliability of each item in the failure-event tree is calculated based on a failure unreliability function. Unreliability and a cost of recovery from each failure event in the failure-event tree are multiplied and results of the multiplication are accumulated in accordance with the tree breakdown to obtain a recovery cost. Preventive maintenance expenses are calculated for preventing failure events. Maintenance timing and technique are decided through comparison between the recovery cost and the preventive maintenance expenses.