A cylinder manufacturing process including the steps of: pressing an annular member of steel onto the outer circumference of one end of a cylindrical tube, which is made of steel and cut to a desired length in advance, and soldering it with a metal solder to form a flange wall; inserting the cylindrical tube portion into that portion of a mold, which has an internal diameter slightly larger than the external diameter of the tube; pressing the tube radially, while being set by fixing the flange wall, to increase the diameter of the tube with a plug which is made movable while being fitted therein and which has a slightly larger diameter than the internal diameter of the tube; and finishing the other end of the tube to product sizes.