PURPOSE: To provide a woodruff key that is able to prevent any fallout and dislocation from a key seat and, what is more, ease of machining and setting work.

CONSTITUTION: A woodruff key 2 consists of a key body 2a made up into semicircular form and a pair of projections 2b installed projectingly in a direction extending a ridgeline equivalent to a semicircualr chord in this key body 2a. In addition, this projection 2b is formed in a part being projected out of a key seat 6 when the woodruff key 2 is inserted into this key seat 6 formed in a turning shaft 4. In this Woodruff key 2 constituted like this, when it is attached to the turning shaft 4, the paired projections 2b cane into contact with a wall surface 4a of the turning shaft 4, preventing any rotation of the Woodruff ket 2 from occurring. Therefore, when a rotary component is attached to the turning shaft 4, even if this component contacts with the l woodruff ket 2 which will in no case cause any dislocation and drop from the key seat 6 at all.