the stop flange 131 thus the deformed riveting portion 41 of the metal panel member 4 is wrapped about the stop flange 13 of the mounting socket 1 to enhance the structural strength of the connection between the mounting socket 1 and the metal panel member 4 further because the outer diameter d 1 of the stop flange 13 is larger than the outer diameter d 2 of the annular step 11 the metal panel member 4 and the mounting socket 1 are firmly secured together and prohibited from separation even if the mounting socket 1 is pulled or forced by an external force the mounting socket 1 will not fall out of the metal panel member 4 assuring a high level of structural stability and shear strength further by means of the grooves 321 in the operating portion 32 of the cap member 3 the cap member 3 can be conveniently and positively pulled to lift the locking member 2 retracting the expanded locking end tip 23 into the inside of the center hole 10 of the mounting socket 1 and forcing the bearing step 231 of the expanded locking end tip 23 to compress the spring member 24 at this time the metal panel member 4 can be attached to a second metal panel member not shown to keep the mounting through hole 40 in vertical alignment with a locating hole in the second metal panel member and then release the pulling force from the operating portion 32 of the cap member 3 enabling the locking member 2 to be forced by the elastic restoring force of the spring member 24 out of the mounting socket 1 and the mounting through hole 40 of the metal panel member 4.