The power plant life cycle costing system for costing a power plant life cycle according to the present invention includes a flow chart generation unit for generating a power plant system flow chart based on specifications and cost information of major components of the power plant stored in a major component specifications storage unit, and plant specifications input from a plant specifications input unit; a power plant planning unit for calculating and generating a power plant life cycle cost, a plant efficiency and an operation plan including a maintenance plan as optimization indexes of a power plant configuration based on information from the flow chart generation unit, specifications and cost information of auxiliary devices of the power plant stored in a piping and device specifications storage unit, and optimization conditions of a power plant configuration input by an optimizing method selection unit; and an optimized result output unit for outputting a calculated value and a planned result calculated and generated by the power plant planning unit.