wherein the first frame includes:

a pair of first and second leg parts in front and rear sides; first and second upper horizontal support bars for connecting upper portions of the first and second leg parts; an upper bracket for coupling the first and second upper horizontal support bars; first and second lower horizontal support bars for connecting lower portions of the first and second leg parts; and a lower bracket for coupling the first and second lower horizontal support bars,

while the second frame includes: a pair of third and fourth leg parts in front and rear sides; a third upper horizontal support bar for connecting upper portions of the third and fourth leg parts; and a third lower horizontal support bar for connecting lower portions of the third and fourth leg parts,

wherein the third upper horizontal support bar is fitted and coupled to the upper bracket while the third lower horizontal support bar is fitted and coupled to the lower bracket such that the first frame and the second frame are cross-assembled in + shape,

wherein the upper bracket is composed of:

a pair of first and second connection pipes into which the first and second upper horizontal support bars are fitted in series, respectively; and a fitting groove which is disposed between the pair of first and second connection pipes and has a structure opened downward such that the third upper horizontal support bar is fitted therein,

while the lower bracket is composed of:

a pair of third and fourth connection pipes into which the first and second lower horizontal support bars are fitted in series, respectively; and a fitting groove which is disposed between the pair of third and fourth connection pipes and has a structure opened upward such that the third lower horizontal support bar is fitted therein.