The present invention relates to an epoxy group-containing silicon compound which is obtained by condensing at least one epoxy group-containing alkoxy silicon compound per se represented by the general formula (1a): R1aSi(OR2)3, wherein R1a denotes a substituent having an epoxy group and R2 denotes an alkyl group having at most 4 carbons, or said compound and at least one substituted alkoxy silicon compound represented by the general formula (1b): R1bSi(OR3)3, wherein R1b denotes an alkyl group having at most 10 carbons, an aryl group or an unsaturated aliphatic residue and R3 denotes an alkyl group having at most 4 carbons, in the presence of a basic catalyst.
