To provide a booster cylinder device that prevents cracks in a cylinder tube in the connecting portion between a first cylinder and a second cylinder, and increases a boost rate of the first cylinder.

A boost cylinder device 1 is a hydraulic booster cylinder formed by connecting a first cylinder portion 2 and a second cylinder portion 3 in series. By pressing a second piston rod 32 of the second cylinder portion 3 into an oil body supplied to the first cylinder portion 2, the pressure of the oil body of the first cylinder portion 2 side is increased to increase the pressure of the output of the first piston rod 22. In addition, a cylinder tube 10 of the first cylinder portion 2 and the second cylinder portion 3 are integrated. In the cylinder tube 10, a partition wall 11 tightly penetrating through the second piston rod 32 is provided between a first piston 21 and a second piston 31.