PURPOSE: To facilitate locking and unlocking and improve durability by providing a knock type locking mechanism in which by holding a cylinder on the advanced position and inserting again a card key so as to a little push the cylinder in, holding of the advanced position of the cylinder is released and the cylinder is moved for return.

CONSTITUTION: By inserting a card key 14 into a card key inserting groove 16, a cylinder 17 is moved to the inner part against a first spring 24. By this movement, a confined latch 18 is advanced to the inner part due to the resiliency of a second spring 30, and engaged with the projecting element 9 of a slider 4 existing in a space part 32. Simultaneously, the condition of the cylinder 17 moved into the inner part is held by means of a knock type locking mechanism 19, the locking condition is held even at drawing the card key 14 out. When the card key 14 is inserted again in this condition, the knock type locking mechanism 19 is disconnected, the cylinder 17 is retreated by the resiliency of the spring 24, the latch 18 is also retreated and detached from the projecting element 9 so as to be in the unlocked condition.