Provided are a curable composition including an amide compound that is represented by Formula (1) below and of which a density of sulfonic acid is 3.9 milliequivalent/g or greater. m represents an integer of 1 or greater, n represents an integer of 2 or greater, L1 represents a m+1-valent linking group, and L2 represents an n-valent linking group. R1 represents a hydrogen atom or an alkyl group, and R2 represents —SO3−M+ or —SO3R3 (R3 represents an alkyl group or an aryl group). Here, in a case where there are plural R2's, not all of the R2's are —SO3R3. M+ represents a hydrogen ion, an inorganic ion, or an organic ion.