The present invention provides a curable composition containing (A) an organic polymer having a silicon-containing group, which can be crosslinked by forming a siloxane bond (100 parts by weight), (B) a diester compound obtained from polyethylene glycol and aliphatic monocarboxylic acid and/or alicyclic monocarboxylic acid (21 - 300 parts by weight), and (C) a curing catalyst (0.01 - 20 parts by weight).