Abstract: The apparatus is for estimating a state of charge of a rechargeable battery charged by a vehicle-mounted power generation apparatus capable of generating a variable output voltage. The estimating apparatus includes a detecting function of detecting a charging/discharging current of the rechargeable battery, and an estimating function of making an estimation of the state of charge on the basis of relationship data defining the relationship among the output voltage, the state of charge, and a convergence value to which the charging/discharging current converges through transient behavior thereof in a state where variation of the output voltage is below a predetermined value after the output voltage is changed.