To provide a charging rate calculation apparatus for secondary battery, which is capable of suppressing the reduction of calculation precision of a charging rate due to detection abnormality of a terminal voltage, and a charging rate calculation method.

A charging rate calculation apparatus includes: a current sensor 4 for detecting a sensor current I; a voltage sensor 3 for detecting a sensor voltage V; an SOC-i calculation section 8 for integrating the sensor current I to calculate a current integration charging rate SOC-i; an open-circuit voltage estimation section 9 for estimating an open-circuit voltage OCV on the basis of the sensor voltage V; an SOC-v calculation section 10 for calculating an open-circuit voltage charging rate SOC-v; an abnormality determination section 11 for determining detection abnormality of a terminal voltage V on the basis of the terminal voltage V; and a charging rate decision section 12 which defines the current integration charging rate SOC-i as a charging rate of a battery 6 when the detection abnormality of the terminal voltage V is determined.