A knock-type writing instrument comprising: a shaft tube having a long hole extending axially in a back peripheral wall thereof; a refill having a writing part at a front end thereof and inserted into the shaft tube from a back end of the shaft tube; a biasing member biasing the refill backward toward the back end of the shaft tube; a knock member inserted into a back part of the shaft tube, wherein its front end abuts a back end of the refill and its back end protrudes from the back end of the shaft tube; a resilient piece connected to the front end of the knock member and extending backward and capable of resiliently deforming in a radial direction of the shaft tube; a lock protrusion formed on a back end of the resilient piece; and a clip having a clip leg connected to the resilient piece and slidably fitted in the long hole, the clip having a front part compressed into contact with or approaching an outer circumference of the shaft tube; wherein a lock hole connected to a back end of the long hole via a middle slit and a back slit connected to a back end of the lock hole to reach the back end of the shaft tube are continuously formed in the back peripheral wall of the shaft tube and extending in the direction of the back end of the long hole; and wherein the knock-type writing instrument is characterized in that the clip leg tapers to form a back end ridge at the back end of the clip leg, the back end ridge adapted to sequentially widen the middle slit and the back slit in their width directions by pushing the clip toward the back end of the shaft tube and out with a pressure greater than a bias force of the biasing member. a shaft tube having a long hole extending axially in a back peripheral wall thereof; a refill having a writing part at a front end thereof and inserted into the shaft tube from a back end of the shaft tube; a biasing member biasing the refill backward toward the back end of the shaft tube; a knock member inserted into a back part of the shaft tube, wherein its front end abuts a back end of the refill and its back end protrudes from the back end of the shaft tube; a resilient piece connected to the front end of the knock member and extending backward and capable of resiliently deforming in a radial direction of the shaft tube; a lock protrusion formed on a back end of the resilient piece; and a clip having a clip leg connected to the resilient piece and slidably fitted in the long hole, the clip having a front part compressed into contact with or approaching an outer circumference of the shaft tube; wherein a lock hole connected to a back end of the long hole via a middle slit and a back slit connected to a back end of the lock hole to reach the back end of the shaft tube are continuously formed in the back peripheral wall of the shaft tube and extending in the direction of the back end of the long hole; and wherein the knock-type writing instrument is characterized in that the clip leg tapers to form a back end ridge at the back end of the clip leg, the back end ridge adapted to sequentially widen the middle slit and the back slit in their width directions by pushing the clip toward the back end of the shaft tube and out with a pressure greater than a bias force of the biasing member.