An apparatus for maintaining plant equipment incorporates an input unit to receive information on targets and ranges of plant equipment to be maintained and a failure-event tree breakdown unit to perform tree-breakdown on failures to be expected from events to be inspected. A failure-unreliability function calculation unit calculates unreliability of each item in failure-event tree breakdown and a failure-derived monetary loss calculation unit multiplies each unreliability and a cost of recovery from each event in accordance with the failure-event tree breakdown. A preventive maintenance expense calculation unit calculates preventive maintenance expenses for preventing failure events and a maintenance decision-making unit decides maintenance timing and technique through comparison between the recovery cost and the preventive maintenance expense.