PURPOSE: To make it possible to ensure a certain shearing length by a piercing punch, by driving the piercing punch radially outward from the inside of a large diameter drilled hole to punch out a small diameter hole continuous coaxially with the large diameter drilled hole.

CONSTITUTION: A carriage is a drill unit is made to approach a cylinder body 1 while spindles are rotated to drill a pair of large diameter holes in the wall of the cylinder body 1. Thereafter, a cam slider 73 is moved to its front end to allow piercing punches 56, 57 to project toward the wall of the cylinder body 1 to punch out small diameter holes. Accordingly, first and second return ports having large and small diameter holes which are continuous coaxially with each other are formed, and simultaneously the curved inner end peripheral sections of the first and second return ports may be formed. At this stage, since the cylinder body 1 is substantially integral with an inserting member 37, the piercing punches act upon the cylinder body 1 in the direction orthogonal to the latter.