A hydraulic forceps system comprising: robotic forceps including a gripper, 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 a 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, and a communication passage through which the first pressure chamber and the second pressure chamber communicate with each other, characterized by: a motor that drives the second piston via a linear motion mechanism; a position sensor used for detecting a position of the second piston; and a control device that controls the motor based on a command position for the first piston, wherein the control device includes: an observer that derives an estimated position of the first piston based on the position of the second piston detected by using the position sensor; and a position controller that derives a target rotational speed of the motor based on a deviation between the estimated position of the first piston and the command position. robotic forceps including a gripper, 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 a 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, and a communication passage through which the first pressure chamber and the second pressure chamber communicate with each other, characterized by: a motor that drives the second piston via a linear motion mechanism; a position sensor used for detecting a position of the second piston; and a control device that controls the motor based on a command position for the first piston, wherein the control device includes: an observer that derives an estimated position of the first piston based on the position of the second piston detected by using the position sensor; and a position controller that derives a target rotational speed of the motor based on a deviation between the estimated position of the first piston and the command position. a motor that drives the second piston via a linear motion mechanism; a position sensor used for detecting a position of the second piston; and a control device that controls the motor based on a command position for the first piston, wherein the control device includes: an observer that derives an estimated position of the first piston based on the position of the second piston detected by using the position sensor; and a position controller that derives a target rotational speed of the motor based on a deviation between the estimated position of the first piston and the command position. an observer that derives an estimated position of the first piston based on the position of the second piston detected by using the position sensor; and a position controller that derives a target rotational speed of the motor based on a deviation between the estimated position of the first piston and the command position.