Solvolysis using mixed solvent of ethylene carbonate (EC) and ethylene glycol (EG) was applied to softwood. In this solvolysis, EG was substituted for hydroxy groups in the lignin molecules. Chain extension reaction of substituted EG occurred during the solvolysis, and DP = 3.0 of oligo EG chains are grafted by 60-min reaction. Linear EG chains are introduced into lignin molecules during the biomass conversion process without additional chemical modification.