A process for producing a polyolefin-based resin composition comprises, in
the first polymerization stage, polymerizing an .alpha.-olefin having 4
to 20 carbon atoms, a styrene or a cyclic olefin in the presence of a
specific catalyst and, in the second polymerization stage, copolymerizing
the obtained polymer with an .alpha.-olefin having 2 to 20 carbon atoms,
a styrene or a cyclic olefin in the presence of a polyene. A
polypropylene composition has a branching parameter a and a branching
index g in specific ranges. The curve showing the change in viscosity
under elongation with time, the degradation parameter D or the content of
a high molecular weight component is specified. The polyolefin-based
resin composition exhibits excellent uniformity and improved workability
in melting due to improved tension in melted condition. The polypropylene
composition exhibits excellent melting elasticity and secondary
workability and provides foamed molded articles, sheets and blow molded
articles.