To provide a base polymer suitable for a tacky hot-melt adhesive which is easily applicable, capable of applying in high precision and excellent in high speed applicability, and has excellent adhesive force even for a combination of same or different kinds of various materials such as polyolefin resins, engineering plastics and inorganic materials.

This low molecular weight and flexible modified-polypropylene is obtained by radically reacting 100 pts.mass of a flexible polypropylene (A) with 0.2-15 pts.mass of an unsaturated carboxylic acid (B) or its derivative and 0.01-2.0 pts.mass of an organic peroxide (C), where the content of the unsaturated carboxylic acid or its derivative is 0.05-15 mass% and the intrinsic viscosity [η] measured in a tetralin solvent at 135°C is 0.05-1.5 dL/g. The resin composition for the tacky hot-melt adhesive uses the resin.