To provide a coupling device for coupling a first tube as a supporting member and a second tube as a supported member, with an angle of about 90 deg. therebetween.

A coupling part 13 composed of a flat base 16 (which is preferably parallelepiped) adapted to be held between two tubes and having two through-holes 18, a first adjustable fastening means 20 surrounding at least a part of one of the tubes and engaged in the through-holes 18, for joining the one of the tubes with the base 16, a second fastening means 22 surrounding at least a part of the other one of the tubes and engaged with the through-holes 18 and the first fastening means, for joining the other one of the tubes with the base 16. With this arrangement, it is possible to easily couple two members having different cross-sectional dimensions with a sufficient aseismatic ability.