Provided is a curable composition which can be used for a sealing material for buildings and has low modulus, high elongation and low viscosity. The curable composition contains an organic polymer (A) that has 1.4 or more reactive silicon groups on average in each molecule and an organic polymer (B) that has less than one reactive silicon group on average in each molecule. The number average molecular weight of the component (B) is smaller than the number average molecular weight of the component (A) by 3,000 or more; and the ratio of the mole number (y) of organic polymers having only one reactive silicon group in each molecule among the organic polymers contained in the component (A) and the component (B) relative to the mole number (x) of organic polymers having two or more reactive silicon groups in each molecule among the organic polymers contained in the component (A) and the component (B), namely (y)/(x) is 5 or less.
