The present invention relates to condensation products of silicic acid derivatives usable especially in optical waveguides and particularly to a condensation product obtained by condensing a silane diol compound (A) of the general formula (1): R12Si(OR2)2 (1) wherein R1 represents a group having at least one aromatic group and having 6 to 20 carbon atoms, and R2 represents hydrogen (H which may also be a heavy hydrogen D), with a modified silane compound (B) of the general formula (2): R3Si(OR4)3 (2) wherein R3 represents an organic group having at least one C═C double bond; and R4 represents a group of CnH2n+1 (n=a number of 1 or 2), in the molar ratio 1.1 to 1.4:1 [(A):(B)], and to a process for producing the same condensation product, as well as to an optical waveguide device fabricated using the same condensation product.