A fastening device for fixing a member to be fixed to a fixing member through a screw, the fastening device includes: a top plate portion; and a pair of leg piece portions respectively suspended from the two side edges of the top plate portion, wherein the top plate portion includes two pairs of first elastic securing pieces disposed opposed to each other for defining a first screw hole formed in the screw, and two pairs of regulating pieces respectively interposed between the first elastic securing pieces for defining a first guide hole having a diameter larger than the first screw hole.