Provided is a curable composition that has low modulus, high elongation and low viscosity, and can be used for building sealing materials. This curable composition includes: an organic polymer (A) containing on average at least 1.4 reactive silyl groups per molecule; and an organic polymer (B) containing on average less than one reactive silyl group per molecule, wherein the number average molecular weight of component (B) is lower than the number average molecular weight of component (A) by at least 3,000, and the ratio (y)/(x) of the number of moles (y) of organic polymers containing only one reactive silyl group per molecule among components (A) and (B) to the number of moles (x) of organic polymers containing at least 2 reactive silyl groups per molecule among components (A) and (B) is not more than 5.