At each Mos Burger store, a limited project to offer "French fries S" as "mountain potatoes" will be held on August 11th, "Mountain Day" (excluding some stores).
A project related to "Mountain", which is the origin of Mos Burger's "MOS (an acronym for Mountain, Ocean, Sun)", is held for one day only on "Mountain Day". If you purchase "French fries S" (220 yen including tax), it will be offered as a heap of "mountain potatoes" that has been increased by 1.5 times for free (including potato S set and moss waiwai set). The cup comes with a sleeve that resembles a mountain day.