To enhance an effect of an anticrime thumb-turn by preventing the easy rotation of a thumb-turn knob, when an intruder performs an illegal unlocking act from the outside by using a wire etc.

This lock makes a tumbler rotated in locking and unlocking directions when a rotational operation is performed while the thumb-turn knob is grasped. In the lock, a rotating shaft member includes a first rotating shaft on the side of the thumb-turn knob, and a second rotating shaft on the side of the tumbler, the back end of which is brought into sliding contact with a leading end of the first rotating shaft. The thumb-turn knob comprises two knob blades; and base ends of the knob blades are each journaled on the outer end of the first rotating shaft. In the spread state of the two knob blades, a locking portion-to-be-engaged, which is formed on each inner surface, is engaged with a locking engaging portion which is formed on an outer end surface of a fixed cylinder. When the two knob blades are rotatively operated by being held in a folded manner between a person's fingers, the locking portion-to-be-engaged is separated from the locking engaging portion; and the two knob blades are put into a rotatively operable state on condition that the first rotating shaft is separated from the back end of the second rotating shaft against a returning force of an urging device or an attracting force of a magnetic body.