elastic plastics or rubber the piston shaft 31 u2032 has a central shaft 311 u2032 inserted in and secured to the lower end 21 u2032 of the inner tube 2 u2032 the piston ring 32 u2032 is fitted between two horizontal arms 312 u2032 of the piston shaft 31 u2032 the outer tube 1 u2032 has an upper end 12 u2032 formed with a reduced inner flange the inner flange has an inner hole for sliding of the inner tube 2 u2032 the inner tube 2 u2032 is inserted in the outer tube 1 u2032 and the outer surface of the piston ring 32 u2032 at the lower end 21 u2032 of the inner 2 u2032 is in touch with the inner wall of the outer tube 1 u2032 the upper end 22 u2032 of the inner tube 2 u2032 is inserted through the inner flange of the upper end 12 u2032 of the outer tube 1 u2032 and pulled upward a pen cap 7 u2032 made of rigid plastics in a stepped cylinder shape is provided the pen cap 7 u2032 has a big end which has a diameter about equal to that of the outer tube 1 u2032 and a small end which has a diameter corresponding to the inner tube 2 u2032 the small end of the pen cap 7 u2032 is fitted in the upper end 22 u2032 of the inner tube 2 u2032 the horizontal arms 312 u2032 of the piston shaft 31 u2032 are.