A fluid damper which is capable of operating while automatically changing the damping force according to the movement of a piston without installing a sensor detecting the displacement of the piston and a controller controlling the supply of electric power. The fluid damper, in which the damping force is automatically changed according to the movement of the piston, comprises a fluid (8) having magnetism, a piston (2) of magnetic material, a cylinder (3) sealing the fluid (8) with magnetism and storing the piston (2), a piston rod (4) passed through the cylinder (3) and supporting the piston (2), magnetic field generating device (6) installed on the outside of the cylinder (3), first yoke member (5) disposed around the cylinder (3), and second yoke member (7) disposed around the piston rod (4) and on the outside of the cylinder (3). Thereby, a magnetic circuit can be partially formed.