The object of this invention is to provide a safety knock-type writing instrument. The writing instrument includes a barrel (10) with a cartridge (50), a gear unit (30), and a knock unit (60). The gear unit (30) is positioned on an exterior of the barrel (10) and rotatably seated on a clip (20). The knock unit (60) is provided in the barrel (10), and includes first and second protrusions (61, 62) to engage with the gear unit (30), thus operating the gear unit. In this writing instrument, when the first protrusion (61) of the knock unit disengages from the gear unit (30), the cartridge is retracted into the barrel. Thus, although a user puts the writing instrument, with the cartridge extended, in a pocket without first retracting the cartridge, the cartridge is automatically retracted into the barrel, thus preventing his / her clothing from being stained by the cartridge.