  We investigate the structure of the molecular cloud complexes (MCCs) as a
group of several giant molecular clouds (GMCs) in the Galaxy. Then, we find
that the mass-size relation which has been reported for the GMCs establishes
well even for the very large MCCs whose size is about 1 kpc. Since the
horizontal size of the MCCs is more than the thickness of the Galactic disk, we
can no longer consider the MCCs to be a sphere. Thus, we construct a structural
model of the MCCs, adopting a rectangular-solid geometry. As a result, our
model explains the observed mass-size relation of the MCCs very well. {}From
the estimated external pressure around the MCCs, we find that they are in a
rough pressure balance with the interstellar medium. Moreover, we find there is
observational deficiency of the MCCs with a large size and surface density.
Then, we suggest that the external pressure has a significant effect on the
structure and evolution of the MCCs. We also discuss the effect of H {\sc ii}
regions in the MCCs.
