Disclosed is a method of preparing a petroleum-alternative bio fuel material such as 5-hydroxymethyl-2-furfural (HMF), 5-alkoxymethyl-2-furfural, levulinic acid alkil ester, etc. through a single process without saccharification, using a catalyst conversion reaction, from galactan that can be massively supplied at low costs and extracted from macroalgae of marine reusable resources.Thus, the macroalgae of the marine biomass resources is used so that a carbon source can be more easily extracted than that of a lignocellulosic biomass resource without a problem of having an effect on grain price like a crop-based biomass.