An objective is to estimate an open circuit voltage of an electric storage device (12) at an early stage. A BMS (20) includes a voltage measurement portion (24)and a CPU (32). The voltage measurement portion (24) is configured to measure voltage values of each cell (14), which is an example of an electric storage device (12). The CPU is configured to: control the voltage measurement portion (24) to measure the voltage values of each cell (14) every predetermined period after charge or discharge of the assembled battery (12) is complete; store the terminal voltages in association with elapsed time since charge or discharge completion time; calculate a variation in terminal voltage per unit time at predetermined elapsed time since the charge or the discharge of the electric storage device (12) is complete based on the terminal voltages measured by the voltage measurement portion (24); and estimate the open circuit voltage based on the predetermined elapsed time, the voltage at the predetermined elapsed time, an
