A gasket insertion method for inserting a gasket into an outer cylinder includes providing an outer cylinder having a base end opening section providing a gasket having a recessed section that is open at a base end of the gasket and that includes a flat bottom surface pushing the gasket into a sleeve configured to be inserted into the outer cylinder, by relatively moving an inserter in a tip end direction with respect to the sleeve while a flat tip end surface of a tip end section of the inserter abuts the bottom surface of the recessed section inserting the sleeve into the outer cylinder through the base end opening section and arranging the gasket inside the outer cylinder by relatively moving the sleeve in a base end direction with respect to the outer cylinder while maintaining relative axial positions of the outer cylinder, the gasket, and the inserter.
