A thinning amount managing apparatus in an embodiment for managing an amount of thinning of a first-stage rotor blade of a steam turbine caused by solid particles contained in working steam for working the steam turbine. The thinning amount managing apparatus includes a display information generation section configured to generate display information for displaying: past thinning amount related information indicating information on a past thinning amount of the first-stage rotor blade from the past to the present calculated based on measured information; and future thinning amount related information indicating information on a future thinning amount of the first-stage rotor blade calculated based on a future operating condition input via a user interface screen and the past thinning amount related information.