A selective cannulation method for a lumenal tissues branch section includes: a first step for guiding an endoscope having a channel to an aperture section of the lumenal tissue a second step for inserting a distal end of an endoscopic treatment instrument through the channel into the aperture section of the lumenal tissue a third step for having the distal end of the endoscopic treatment instrument make contact with a pipe wall extending from the aperture section of the lumenal tissue to the branch section at two opposed points on the pipe wall by deforming the distal end of the endoscopic treatment instrument a fourth step for sliding and moving the distal end of the endoscopic treatment instrument to the branch section while having the distal end of the endoscopic treatment instrument make contact with the pipe wall and a fifth step for inserting the distal end of the endoscopic treatment instrument into a predetermined branch lumen divided from the branch section.
