PURPOSE: To reduce the hysteresis of designation by forming a structure in which a contact overcrowded part between wires of a hair spring is so set as to be eccentrically deviated toward an outer end locking part with a rotary shaft as a boundary.

CONSTITUTION: A crank-shaped bent part having a first bent 14, a second bent 15 and a third bent 16 is formed at the locking end of a housing of a spiral hair spring 12, and further a pair of protrusions 17A, 17B in contact with a first bent side end and a third bent side end of the bent 15 to energize to lock the spring 12 are formed in the housing. At the time of mounting an instrument of the spring 12, when the inclination of the bent 15 is controlled by the protrusions 17A, 17B and a needle is rotated against elasticity of the spring and wound to reduce its diameter, the wires of the spring is not brought into contact with one another, and hysteresis of designation by an operation of the needle is reduced.