A radiation shielding putty-like composition for use in effectively preventing radiation leakage by filling openings of any shape therewith is characterized in that it comprises 100 weight parts of a binder having at 25°C a viscosity in the range of not less than 1 x 10 4 centistokes to an unworked penetration of not less than 100, 1,200-4,000 weight parts of an inorganic powder having a density of not less than 5 and a particle size such that at least 80% thereof passes a 100-mesh sieve and 0.1-50 weight parts of a fiber having a diameter of 0.05-50 µm and an average length of 0.1-50 mm and has a density of not less than 4 and a consistency of 1-25.