The present invention relates to a method for producing highly-concentrated L-amino acid and riboflavin simultaneously, and a microorganism for simultaneously producing L-amino acid and riboflavin. Specifically, the present invention relates to a modified microorganism for producing L-lysine or L-threonine, and riboflavin simultaneously, wherein the microorganism belonging to Corynebacterium sp. capable of producing L-lysine or L-threonine is modified by enhancing the activity of an enzyme family expressed by a rib operon which contains riboflavin biosynthesis gene family. Also, the present invention relates to a method for the simultaneous production of L-lysine or L-threonine, and riboflavin using the modified microorganism, and relates to a formulation or granular formulation, feed, and feed additive, containing L-lysine or L-threonine, and riboflavin produced from a culture medium of the modified microorganism.