An estimation apparatus has a controller which estimates an internal reaction of a secondary battery. The controller calculates a voltage drop amount due to an internal resistance of the secondary battery by using an expression (I): Δ ⁢ ⁢ V = RT α ⁢ ⁢ β ⁢ ⁢ F ⁢ arc ⁢ ⁢ sinh ( - R r ⁢ I RT α ⁢ ⁢ β ⁢ ⁢ F ) - R d ⁢ I ( I ) where ΔV represents the voltage drop amount, R represents a gas constant, T represents a temperature, α represents an oxidation reduction transfer coefficient (α=0.5) of an electrode, β represents a correction coefficient (0