An estimation system, includes: a secondary battery; a monitoring device that detects a voltage and a current of the secondary battery; and a processor that estimates a full charge capacity of the secondary battery using a detection result of the monitoring device. The processor calculates a first charging rate of the secondary battery using a current integration amount in charging, discharging, or charging and discharging of the secondary battery and using a full charge capacity of the secondary battery that was estimated last time; calculates a second charging rate of the secondary battery using an open circuit voltage of the secondary battery, when a predetermined time has elapsed without charging and discharging since the first charging rate is calculated; and performs correction, when a magnitude of a difference between the first and second charging rates is larger than a threshold, on the full charge capacity based on the difference.