A problem to be solved of the present invention is to provide a drug formulation and a method of administering an active ingredient which allow the active ingredient to be delivered evenly into the site of action in the skin with high efficiency while ensuring stability of the active ingredient over a long time, and which are easily handled and are less stressful for patients. Mean for solving the problem is a microneedle assembly formulation for skin treatment comprising a platform and a plurality of conical or pyramidal microneedles formed on the platform containing a base composed of a bio-soluble and thread-forming polymer substance and an objective substance retained in the base, wherein the objective substance is a substance effective for prevention or treatment of skin senescence, or treatment of skin scar.