A method for putting together a down-the-hole drilling apparatus for plastic pipe drilling. The down-the-hole drilling apparatus has a drilling device for carrying out drilling by using a drilling unit with a plastic pipe, which drilling unit includes in its drilling head a drilling arrangement. The plastic pipe is arranged to be pulled into the hole to be drilled by a power influence directed thereto from the drilling unit, being coupled with the second drilling means in a manner transmitting power at least in the longitudinal direction of the drilling device. The drilling unit and the plastic pipe are coupled by a skirt part that exists at an opposite end of the drilling arrangement, with respect to the drilling surface of the drilling unit, which skirt part has an internal protrusion assembly in radial direction.