A retractable writing instrument having a dryness prevention device is disclosed. The retractable writing instrument includes a casing (10), which has a nib hole therein, a cartridge (20), which has a nib coupling protrusion for coupling a nib (24) to a nib extension tube, and a spring for providing elastic restoring force. The writing instrument further includes an Oring member (50), which is coupled to the nib extension tube, and an openable ring (40), which is rotatably coupled to the Oring member (50). When a knock part is pushed, the openable ring (40) is rotated, such that a through passage is opened, and the nib is protruded outside through the nib hole. When the cartridge is moved backwards, the nib coupling protrusion of the cartridge holds the Oring member and is moved backwards along with the Oring member, thereby rotating the openable ring, thus closing the through passage.