To obtain the subject stable acid anhydride large in nonlinear optical constant, low in light scattering, and useful for optical waveguides, wavelength transform elements, etc., by reaction a specific azo group-contg. dihydroxybenzene with a trimellitic anhydride compound.

This acid anhydride is obtained by condensation reaction of (A) a compound of the formula [X is H, a halogen, CN, NO2 or a lower alkyl- substituted tertiary amino (R); Y is H, a halogen, CN or NO2 when X is R; otherwise, being H or R] {e.g. 2,4-dihydroxy[(4-nitrophenyl)azo]azobenzene} with (B) trimellitic anhydride (monochloride) followed by precipitation. Furthermore, as necessary, the precipitate is dissolved in acetic anhydride under heating followed by cooling to crystallize.