In order to provide an operation and maintenance planning aiding system for a power generation installation which prepares an operation plan for a plurality of power generation units by making use of actual plant data and based on a total judgement including a variety of circumstances of the machine and apparatus or the parts thereof in the power generation units, in the system the plurality of power generation units 41, 42, 51 and 52, a power supply command center 3 and a service center 1 are arranged and connected via a communication network 6, the service center 1 obtains the plant data via the communication network 6 from the plurality of power generation units 41, 42, 51 and 52, calculates in real time a power generation efficiency of a concerned power generation unit for every plurality of power generation units 41, 42 51 and 52 by making use of the obtained plant data and design data of the concerned power generation unit and prepares an operation and maintenance plan for each of the power generation units based on the calculated power generation efficiency.