NOVELTY - A curable composition is one-component composition containing an organic polymer (A) having a reactive silicon group, and hollow particles (B). The true specific gravity of hollow particles is 0.025-0.1. The ignition residue of hollow particle is 20% or less. The average particle diameter of the hollow particles is 30-60 mu m. The content of compound having boiling point of 20-40 degrees C, is 50 wt.% or more with respect to total content of foaming agent.    USE - Curable composition is used for hardened material for sealing material, and adhesive agent for tiles and floor (all claimed). Uses include but are not limited to solar cell backside sealing material, elastic adhesive agent, spray-type sealing material, crack repair material, casting material, medical rubber material, medical device sealing material, food packaging material, electroconductive material for electromagnetic wave shielding, thermally conductive material, and motor-vehicle component.    ADVANTAGE - The curable composition has excellent thread breakage property and removability. The hardened material formed of curable composition has excellent flexibility and adhesiveness.    DETAILED DESCRIPTION - A curable composition is one-component composition containing an organic polymer (A) having a reactive silicon group, and hollow particles (B) obtained by carrying out heating expansion of thermally expansible microcapsule having outer shell containing thermoplastic resin obtained by polymerizing a polymerizable component containing nitrile group monomer and vaporizable foaming agent, and heating. The true specific gravity of hollow particles is 0.025-0.1. The ignition residue of hollow particle is 20% or less. The average particle diameter of the hollow particles is 30-60 mu m. The content of compound having boiling point of 20-40 degrees C, is 50 wt.% or more with respect to total content of foaming agent. An INDEPENDENT CLAIM is included for hardened material, which is obtained by hardening the curable composition.