To rovide a key cylinder mechanism having durability against breaking by insertion of a screwdriver or the like.

A center hole in a liner 2 which serves as a key hole for a key cylinder is cylindrical in shape and has an inside diameter of 10 mm or more, so the key cylinder mechanism has higher rigidity for the key hole and has a wider open space above the key hole than conventional products. Thus, the key cylinder mechanism can prevent tampering by insertion of a commercially available screwdriver into the key hole to break the mechanism.