The present invention is a curable composition comprising a component (A), a component (B), a component (C), and a component (D), the curable composition comprising the component (A) and the component (B) in a mass ratio (component (A):component (B)) of 100:0.3 to 100:20, the component (A) being a silane compound (co)polymer that is represented by a formula (a-1) or a formula (a-2), (CHR1X0-D-SiO3/2)m(R2SiO3/2)n(CHR1X0-D-SiZ1O2/2)o(R2SiZ2O2/2)p(CHR1X0-D-SiZ32O1/2)q(R2SiZ42O1/2)r   (a-1) (R3SiO3/2)s(R3SiZ5O2/2)t(R3SiZ52O1/2)u   (a-2) the component (B) being fine particles having an average primary particle size of more than 0.04 μm and 8 μm or less, the component (C) being a silane coupling agent that comprises a nitrogen atom in its molecule, and the component (D) being a silane coupling agent that comprises an acid anhydride structure in its molecule, and an optical element-securing composition, and a cured product, and an optical element-securing material, and a method for using the curable composition, and an optical device.