To provide an instrument and a method for measuring precisely the charge capacity of a charge accumulation part for transferring and receiving electric power to and from a motor generator for driving a vehicle.

A control part acquires a measured voltage V1 at the time t1 when the vehicle is brought into an IG-ON state in an A point, and finds a time integration value of a measured current during a period from the time t2 when the vehicle starts traveling from the A point toward a B point, up to the time t3 when the vehicle reaches the B point and is brought into a nontravel state. The control part acquires thereafter a measured voltage V4 at the time t4 when the vehicle is brought into the IG-ON state, and calculates the charge capacity, based on the measured voltage V1, the measured voltage V4 and the time integration value of the measured current.