The present invention is: a curable composition comprising a component (A) and a component (B) in a mass ratio (component (A):component (B)) of 100:0.3 to 100:90, the component (A) being a silane compound polymer that is represented by a formula (a-1), wherein R1 is a group selected from an alkyl group having 1 to 20 carbon atoms, a cycloalkyl group having 3 to 10 carbon atoms, and a substituted or unsubstituted aryl group having 6 to 20 carbon atoms, Z is a hydroxyl group, an alkoxy group having 1 to 10 carbon atoms, or a halogen atom, m is a positive integer, and n and o are independently 0 or a positive integer, provided that a plurality of R1 are either identical or different, and a plurality of Z are either identical or different, and the component (B) being a silane coupling agent that comprises an isocyanurate skeleton in its molecule. (R1SiO3/2)m(R1SiZO2/2)n(R1SiZ2O1/2)o  (a-1)