The invention provides a writing instrument which makes it possible to automatically and efficiently mount a means for sealing spaces between a holder and in-holder fitting parts of a lead holder and a tail plug without depending on manpower to thereby greatly improve the assembling efficiency of the writing instrument, and a method for providing a sealing means in a writing instrument. [Solution] A writing instrument provided with a lead holder (3) and/or a tail plug (11) that are fitted into a holder (1) containing wadding or ink with a sealing means therebetween, wherein the sealing means is ring-shaped seals (8, 13) that are formed by secondary molding in in-holder fitting parts (7, 12) of the primarily molded lead holder (3) and tail plug (11) by a double molding method. Belt-shaped gates (9, 14) are provided to be continuous with the ring-shaped seals (8, 13) so as to surround the peripheral surfaces of the in-holder fitting parts (7, 12), and recessed parts (10, 15) housing the ring-shaped seals (8, 13) and the belt-shaped gates (9, 14), respectively, are circumferentially provided at the peripheral surfaces of the in-holder fitting parts (7, 12).