To prevent the inner peripheral surface of a housing hole from being scratched, and to extend the life of a nib and that of a case by providing a slit extending in a direction except that extending along the axial line of the housing hole housing the nib, and exhausting air.

A housing hole 14 has been provided in a cylindrical nib sleeve 13, which extends in the axial direction of the nib sleeve 13. On the outer peripheral surface of the nib sleeve 13, a slit 15 is provided, which extends with an inclination to the axial line of the housing hole 14 in the peripheral direction of the nib sleeve 13. Before force-fitting a nib 16 into the housing hole 12 of a case 11, the housing state of the nib 16 can be inspected through the slit 15. When the nib 16 is shifted to the axial line of the housing hole 14, since the diameter of the nib sleeve 13 has been expanded, the nib 16 can be easily taken out and can be inserted again. When a stock 23 is formed, air stayed behind in a gap between the stock 23 and the formed part 17a of the stock, is exhausted outside through a communicative hole 18 and the slit 15.