To protect an automobile from theft by preventing an insertion spring part from curling and also sliding.

In a retainer clip 1, a pair of insertion springs 2 having a U-shaped flat surface is formed in such a manner as to be split into the inside part 4 of the retainer clip 1 and the outside part 5 thereof, respectively, by a slit 3. The inside part 4 is formed in such a manner as to have approximately the same width as the upper part of a cylinder slit mounted on a pair of a rotation stopping projections formed at the end part of the flange part of a key cylinder. The retainer clip 1 is formed with a protrusion 10 made to press- contact the upper part of the slit 3, an inclined part 11 contacting the protrusion 10 continuously and inclined downward in a slating direction, and a projecting part 12 engaged with the projection of the key cylinder.