A predicted corrosion amount of a painted or unpainted atmospheric corrosion resistant steel is calculated by using extrinsic corrosion information including weather observation data, an amount of airborne salt, and an amount of sulfur oxide in a planned location for use where the atmospheric corrosion resistant steel is to be used, and intrinsic corrosion information on components of the atmospheric corrosion resistant steel. The weather observation data on this occasion preferably includes the annual wetness time, annual mean wind speed, and annual mean temperature. Moreover, it is preferable to calculate a corrosivity index, estimate a first-year corrosion amount of the atmospheric corrosion resistant steel and a rust stabilization index from the corrosivity index, and calculate a corrosion amount accumulated over time.