The object is to provide a gasket insertion method and a gasket which are capable of suppressing inclination of the gasket with respect an outer cylinder. A gasket (10) is provided with a recessed section (30) having a flat bottom surface (34). A gasket insertion method includes a pushing step of pushing the gasket (10) into a sleeve (46) in a state where a flat tip end surface (41) provided to a tip end section (40) of an inserter (36) and the bottom surface (34) of the recessed section (30) are abutted against each other, an insertion step of inserting the sleeve (46) into an outer cylinder (18), and an arrangement step of placing the gasket (10) inside the outer cylinder (18) by relatively moving the sleeve (46) in a base end direction with respect to the outer cylinder (18).
