To fabricate the cylinder unit in a short time and a low cost, wherein a diameter in an axis direction is different and a thickness is practically the same as that of a raw material cylinder.

According to the cylinder unit forming method, in a cylindrical shape framework 5, inner diameters of which is different in the axis direction, a steel pipe (a raw material pipe) 9 having its both ends blocked with No.1, No.2 blocking means is made compressive deformed in the axis direction. Simultaneously, the steel pipe 9 is swelled towards a radial direction by a hydraulic pressure fed inside the pipe, resulting in that the steel pipe 9 is formed in a shape along a form face of the framework 5.