To provide a thumb-turn capable of very easily setting on a door.

A steady liner 1 is fixed with a plurality of mounting screws 12 and 12 so that it can be called to the inside of the door 4, a thumb-turn seat 14 forming a supporting cylinder consisting of a plurality of cup-like divided supporting pieces capable of covering a part projected to the indoor side of the steady liner is covered on the part thereof, an interlocking claw 18 is formed on the inside of the free end of each divided supporting piece, in the meantime, a thumb-turn shaft 21 integrally connecting a knob 19 is inserted in an inner cylinder section of the supporting cylinder to lock it by the interlocking claws 18 and 18, and on the other hand, engaging recess sections 11 are formed in a plurality of places in either one of the outer circumferential surface of a large diameter section of the steady liner and the inner circumferential surface of a thumb-turn seat circular bank section connected to the steady liner, engaging projection sections 23 are formed in the other.