PURPOSE: To prevent any drop in the strength of a key and its malfunction form occurrding by defining at least and endmost projection among plural pieces of key code projections installed in the key as a complete insertion checking code, while installing a checking tumbler.

CONSTITUTION: A complete insertion checking projection 78 is installed in at least an endmost projection among plural pieces of key code projections 49 installed in a key K. On the other hand, a complete insertion checking tumbler is installed in a plug 36. When a key K is inserted into a key hole, the tumbler pushed by the key code projection installed in the key moves in direction of making the plug 36 rotatable. The complete insertion checking tumbler also moves in direction of making the plug rotatable after being pushed by the checking projection 78 at a time when the key is completely inserted. With this constitution, a key code conversion is made so as to be performable without fail.