PURPOSE: To make it possible to unlock by turning an inner cylinder when a key is engaged with a tumbler in the inner cylinder and the key is displaced in the vertical direction and positioned between specific lines.

CONSTITUTION: A turnable inner cylinder 20 has a hollow cylindrical part in an outer cylinder 10. A tumbler 30 is formed in such a fashion that it may be displaced in the vertical direction LO in the inner cylinder 20. An opening is installed to a front end 15 of the hollow cylindrical part 14. A front shaft of the inner cylinder is fitted thereinto, and what is more, an engagement part 42 of a key 40 is formed so that it may be inserted. When it is unlocked by pressing the key 40, the tumbler 30 is placed in contact with cavities 14A and 14B formed in the front end 15 of the outer cylinder 10 in the beginning, but when the key 40 is inserted so as to engage itself with the tumbler 30, the tumbler 30 is forcibly pushed and located between a first shear line S1 and a second shear line S2. It is possible to unlock the key by turning the key 40 in the state described above.