Provided is a knock type writing instrument provided with a simple mechanism allowing a stable fretting operation or the like. A knock type writing instrument 1 is provided with: a shaft cylinder 2; a refill 5 arranged inside the shaft cylinder 2; a spring 6 biasing the refill 5 backward; an operation part 20 pressed forward against a biasing force of the spring 6 in a knocking operation; and a main rotor 30, and is switchable between a writing state and a non-writing state with the knocking operation. The knock type writing instrument 1 is further provided with: a knock locking member 50 movable forward and backward inside the shaft cylinder 2 due to gravity; and a locking part 60 provided on a side of the shaft cylinder 2 and capable of being locked with the knock locking member 50. When a front end of the shaft cylinder 2 is directed upward, the knock locking member 50 moves backward to be locked with the locking part 60 to prevent forward movement of the operation part 20.