Even if atrial-appendage tissue is damaged due to an excessive force unintentionally exerted during ligation of the atrial appendage, the effect of the damage is minimized. Provided is an atrial-appendage ligation treatment tool that is used, together with an atrial-appendage ligation tool having a ligation part, to perform atrial-appendage ligation treatment. The atrial-appendage ligation treatment tool includes: a compressing portion that is introduced, separately from the ligation part, to a cardiac sac through a sheath penetrated through a pericardium and that compresses the atrial appendage from outside to occlude an internal space therein; and a shaft that is disposed in the sheath in an inserted manner and that supports, at a distal end thereof, the compressing portion.