To provide a small booster cylinder capable of passing an operation axis tool for liquid flow or through a through-hole that passes through a piston rod.

The booster cylinder comprises a primary-side piston 3a moved by a hydraulic liquid flowing into a primary-side cylinder 1a, a secondary-side piston 3b having a cross section smaller than the primary-side piston 3a and integrally and movably formed with the piston 3a, an output piston 4 moving in a secondary-side cylinder 1b, a booster chamber 9 sealed with liquid and formed with the secondary-side opposed-piston 3b and the output piston 4 with its pressure being increased by the internal movement of the secondary-side piston 3b, and a rod, with a through-hole 6a passing through the axial center, passing through the primary-side piston, rod of the secondary-side piston, output piston, and axial center of flanges at both ends of the cylinder.