The present invention relates to a method of producing biofuel, more specifically a method of producing biofuel comprising the steps of generating monosugars from marine algae, or from polysaccharides extracted from marine algae by treating the marine algae or the polysaccharides with a hydrolytic enzyme and/or a hydrolytic catalyst; and fermenting the monosugars using a microorganism to produce biofuel. The method of producing biofuel of the present invention solve the problem of raw material suppliance since it uses marine algae as a raw material for biomass, and reduce the production costs by excluding lignin eliminating process that has been required by the conventional method using wood-based raw materials, resulting in economic and environmental advantages.