Abstract: A hydraulic booster using a variable-volume piston includes a main cylinder configured such that, if a fluid introduced into the upper portion of a main piston is pressurized by a pressurizing means, the fluid in the lower portion of the main piston is output. A guide moves downwards separately from the main piston and then moves to the original position by a return means. A variable-volume piston is integrally assembled on the guide such that the volume thereof increases during a downward movement and decreases during an upward movement, thereby changing the volume of the upper side of the main piston. A fluid storage-and-supply unit connects to fluid channels in the upper and lower portions of the main cylinder. This configuration boosts the pressure of the fluid discharged through the lower portion of the main cylinder due to a volume change caused by upward/downward movements of the variable-volume piston.