The present invention relates to a pressure-sensitive adhesive composition comprising: a monomer mixture comprising the following (a1), (a2) and (a3) or partially polymerized product thereof; and thermal-expandable fine particles: (a1): alkyl (meth)acrylate monomer having 4 to 12 carbon atoms in its alkyl moiety and having a glass transition temperature when formed into a homopolymer of lower than 0° C.; (a2): monomer having at least one nitrogen atom and one ethylenically unsaturated bond in its molecule; and (a3): monomer having one ethylenically unsaturated bond in its molecule and having a glass transition temperature when formed into a homopolymer of 0° C. or higher (excluding (a2) above).