A robot hand has a pipe which is sufficiently flexible to allow a change in the cross-sectional area thereof according to the pressure of a fluid, thereby permitting a higher degree of freedom of the layouts of the pipe and a master cylinder connected through the pipe. Moreover, considerations are given to a change in the cross-sectional area of the pipe caused by a change in the pressure of the fluid in the pipe, thus making it possible to measure the position of a slave piston with high accuracy.