To provide a technical means enabling the easy alignment of the axis of the female screw with the axis of the male screw of a frame connection device, preventing frames from being damaged when the frames are joined to each other, and capable of changing screw diameters according to the sizes of the frames.

Each of round nut receiving tools 40a, 40b inserted into the second holes 25, 26 of the upper frame 20 comprises a cylindrical member 44 having a bolt insertion hole 43 extending through the side surface and a plate nut member 46 having the female screw 45 generally concentric to the bolt insertion hole 43 of the cylindrical member. When the male screws 51 of bolt connection tools 50a, 50b are inserted into the through holes 31, 32 of the vertical frame 30 and the first holes 22, 23 of the upper frame and screwed into the female screws of the round nut receiving device, even if the axes of these screws are angularly displaced, the round nut receiving tool is rotated by that displacement angle in the second hole to act an automatic aligning action to align the female screw axis 47 of the round nut receiving tool with the male screw axis 48 of the bolt connection tool.