A power storage element is provided that includes, in a positive electrode or a negative electrode, an active material which causes a plurality of electrochemical reactions in accordance with the process of change during charging and discharging and which exhibits hysteresis between a power storage amount-voltage charging (first) property and a power storage amount-voltage discharging (second) property. A BMU is provided with a first estimation unit which estimates a power storage amount-voltage charging property or/and a power storage amount-voltage discharging property referenced when estimating the power storage amount, on the basis of an upward voltage and a downward voltage which are respectively more than and less than a first threshold value, acquired from the first property, the second property, and the charging-discharging history.