An electro hydrostatic actuator system for lifting or lowering a leg of an aircraft, comprising: a hydraulic circuit (101, 102, 103) including a hydraulic actuator configured to be attached to the leg to lift or lower the leg, at least one electric hydraulic pump (3, 31, 32) configured to discharge an hydraulic fluid supplied to the hydraulic actuator, and a hydraulic path connecting the hydraulic actuator j and the electric hydraulic pump (3, 31, 32) to each other; a pressure sensor (38, 83) configured to output a measurement signal corresponding to a pressure of the hydraulic fluid in the hydraulic circuit (101, 102, 103) during operation of the electric hydraulic pump (3, 31, 32); a temperature sensor configured to output a measurement signal corresponding to a temperature of the hydraulic fluid; and a control unit configured to output a control signal for operating the electric hydraulic pump (3, 31, 32), wherein the hydraulic circuit (101, 102, 103) includes a pressure increasing element configured to increase the pressure of the hydraulic fluid discharged from the electric hydraulic pump (3, 31, 32), the control unit performs health monitoring regarding performance of the electric hydraulic pump (3, 31, 32) based on the pressure of the hydraulic fluid based on the measurement signal of the pressure sensor (38, 83), the temperature of the hydraulic fluid based on the measurement signal of the temperature sensor, and a speed of the electric hydraulic pump (3, 31, 32) during operation of the electric hydraulic pump (3, 31, 32), and the pressure increasing element is the hydraulic actuator, characterized in that the control unit calculates an actual discharge flow rate of the electric hydraulic pump (3, 31, 32) based on the pressure of the hydraulic fluid, the temperature of the hydraulic fluid, and the speed of the electric hydraulic pump (3, 31, 32) during leg lifting by the hydraulic actuator, a time required for the leg lifting by the hydraulic actuator, and an amount of the hydraulic fluid required for the leg lifting by the hydraulic actuator, and calculates a volumetric efficiency of the electric hydraulic pump (3, 31, 32) from the calculated actual discharge flow rate and an ideal discharge flow rate of the electric hydraulic pump (3, 31, 32) determined from the speed of the electric hydraulic pump (3, 31, 32). a hydraulic circuit (101, 102, 103) including a hydraulic actuator configured to be attached to the leg to lift or lower the leg, at least one electric hydraulic pump (3, 31, 32) configured to discharge an hydraulic fluid supplied to the hydraulic actuator, and a hydraulic path connecting the hydraulic actuator j and the electric hydraulic pump (3, 31, 32) to each other; a pressure sensor (38, 83) configured to output a measurement signal corresponding to a pressure of the hydraulic fluid in the hydraulic circuit (101, 102, 103) during operation of the electric hydraulic pump (3, 31, 32); a temperature sensor configured to output a measurement signal corresponding to a temperature of the hydraulic fluid; and a control unit configured to output a control signal for operating the electric hydraulic pump (3, 31, 32), wherein the hydraulic circuit (101, 102, 103) includes a pressure increasing element configured to increase the pressure of the hydraulic fluid discharged from the electric hydraulic pump (3, 31, 32), the control unit performs health monitoring regarding performance of the electric hydraulic pump (3, 31, 32) based on the pressure of the hydraulic fluid based on the measurement signal of the pressure sensor (38, 83), the temperature of the hydraulic fluid based on the measurement signal of the temperature sensor, and a speed of the electric hydraulic pump (3, 31, 32) during operation of the electric hydraulic pump (3, 31, 32), and the pressure increasing element is the hydraulic actuator, characterized in that the control unit calculates an actual discharge flow rate of the electric hydraulic pump (3, 31, 32) based on the pressure of the hydraulic fluid, the temperature of the hydraulic fluid, and the speed of the electric hydraulic pump (3, 31, 32) during leg lifting by the hydraulic actuator, a time required for the leg lifting by the hydraulic actuator, and an amount of the hydraulic fluid required for the leg lifting by the hydraulic actuator, and calculates a volumetric efficiency of the electric hydraulic pump (3, 31, 32) from the calculated actual discharge flow rate and an ideal discharge flow rate of the electric hydraulic pump (3, 31, 32) determined from the speed of the electric hydraulic pump (3, 31, 32). calculates an actual discharge flow rate of the electric hydraulic pump (3, 31, 32) based on the pressure of the hydraulic fluid, the temperature of the hydraulic fluid, and the speed of the electric hydraulic pump (3, 31, 32) during leg lifting by the hydraulic actuator, a time required for the leg lifting by the hydraulic actuator, and an amount of the hydraulic fluid required for the leg lifting by the hydraulic actuator, and calculates a volumetric efficiency of the electric hydraulic pump (3, 31, 32) from the calculated actual discharge flow rate and an ideal discharge flow rate of the electric hydraulic pump (3, 31, 32) determined from the speed of the electric hydraulic pump (3, 31, 32).