It is an object of the present invention to provide a gasket insertion method and a gasket which can suppress inclination of a gasket with respect to an outer cylinder. The gasket (10) is provided with a recess (30) having a flat bottom surface (34). In the gasket insertion method, the gasket (10) is inserted in a state where the flat distal end surface (41) provided at the distal end portion (40) of the inserter (36) and the bottom surface (34) of the recessed portion (30) Inserting the sleeve into the sleeve, inserting the sleeve into the sleeve, inserting the sleeve into the sleeve, inserting the sleeve into the sleeve, inserting the sleeve into the sleeve, And placing the gasket (10) in the outer cylinder (18) as the gasket (10) is moved.
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