PURPOSE: To provide a locking structure, by which a link plate (engaging piece) installed in a heater case can be easily removed without any damage so as to be reused, for the link plate.

CONSTITUTION: A link plate 1 is provided with a rigid body part 5, which is inserted in the inner circumference of a boss part 3 arranged in a heater case 2, and a plurality of engaging pieces 6, which are connected to the rigid body part. 5, and on the inside of the heater case 2, engaging jaws 6d arranged at the tips of the engaging pieces 6 are engaged with an opening circumferential edge part in the boss part, 3 so as to prevent the link plate 1 from coming off from the boss part, 3. In each of the engaging pieces 6, which are provided with the engaging jaws 6d and are joined to the rigid body part 5 via a joint part 6a elastically deformably in the radius direction toward the rigid body part 5, one end side 6b is extended from the joint part 6a to the outside of the heater case 2, while the other end side 6c is extended from the joint part 6a to the inside of the heater case 2, and the engaging jaw 6d is arranged at the tip of the other end side 6c.