Provided are a fluid actuator operating smoothly, and an operation assistance device and a startup-operation assistance device that have the fluid actuator. The fluid actuator has a tube that is swollen as fluid is supplied through a fluid supply tube and also has a sleeve that surrounds the tube and whose opposite ends are fixed to opposite ends of the tube. The sleeve is formed by braiding first fiber cords and second fiber cords along the outer circumferential surface of the tube. The first fiber cords are wound left handed around the tube, in the longitudinal direction of the tube, and the second fiber cords are wound right-handed around the tube, in the longitudinal direction. The sleeve has a first region in which the first fiber cords and the second fiber cords are wound while being made to cross at a first angle, and also has a second region in which they are wound while being made to cross at a second angle different from the first angle.
