A hydraulic forceps system comprising: robotic forceps whose gripper is opened and closed by utilizing hydraulic pressure of a hydraulic fluid, the robotic forceps including: a first piston coupled to the gripper; a first cylinder accommodating the first piston and forming a first pressure chamber together with the first piston, the first pressure chamber being filled with the hydraulic fluid; a second piston; a second cylinder accommodating the second piston and forming a second pressure chamber together with the second piston, the second pressure chamber being filled with the hydraulic fluid, a communication passage through which the first pressure chamber and the second pressure chamber communicate with each other, and a motor that drives the second piston via a linear motion mechanism; a pressure sensor that detects a pressure of the hydraulic fluid; a control device that calculates a current gripping force of the gripper based on the pressure of the hydraulic fluid detected by the pressure sensor; a monitor that displays the current gripping force; and a position sensor used for detecting a position of the second piston, wherein the control device includes: an observer that derives an estimated position of the first piston based on the pressure of the hydraulic fluid detected by the pressure sensor and the position of the second piston detected by using the position sensor; and a gripping force calculator that calculates the current gripping force based on the estimated position of the first piston and the pressure of the hydraulic fluid detected by the pressure sensor. a first piston coupled to the gripper; a first cylinder accommodating the first piston and forming a first pressure chamber together with the first piston, the first pressure chamber being filled with the hydraulic fluid; a second piston; a second cylinder accommodating the second piston and forming a second pressure chamber together with the second piston, the second pressure chamber being filled with the hydraulic fluid, a communication passage through which the first pressure chamber and the second pressure chamber communicate with each other, and a motor that drives the second piston via a linear motion mechanism; a pressure sensor that detects a pressure of the hydraulic fluid; a control device that calculates a current gripping force of the gripper based on the pressure of the hydraulic fluid detected by the pressure sensor; a monitor that displays the current gripping force; and a position sensor used for detecting a position of the second piston, wherein the control device includes: an observer that derives an estimated position of the first piston based on the pressure of the hydraulic fluid detected by the pressure sensor and the position of the second piston detected by using the position sensor; and a gripping force calculator that calculates the current gripping force based on the estimated position of the first piston and the pressure of the hydraulic fluid detected by the pressure sensor. a pressure sensor that detects a pressure of the hydraulic fluid; a control device that calculates a current gripping force of the gripper based on the pressure of the hydraulic fluid detected by the pressure sensor; a monitor that displays the current gripping force; and a position sensor used for detecting a position of the second piston, wherein the control device includes: an observer that derives an estimated position of the first piston based on the pressure of the hydraulic fluid detected by the pressure sensor and the position of the second piston detected by using the position sensor; and a gripping force calculator that calculates the current gripping force based on the estimated position of the first piston and the pressure of the hydraulic fluid detected by the pressure sensor. an observer that derives an estimated position of the first piston based on the pressure of the hydraulic fluid detected by the pressure sensor and the position of the second piston detected by using the position sensor; and a gripping force calculator that calculates the current gripping force based on the estimated position of the first piston and the pressure of the hydraulic fluid detected by the pressure sensor.