Abstract: This invention relates to a process for readily and efficiently preparing L-rhamnose with the use of a marine alga belonging to the family Monostromaceae, Ulvales.According to the process of the present invention, rhamnan sulfate is extracted from a marine alga belonging to the family Monostromaceae such as Monostroma nitidium Wittrock and the obtained extract is hydrolyzed by adding acid(s) thereto followed by heating or treating thereof with a cation exchange resin followed by heating to thereby give a solution containing free L-rhamnose.Since the solution thus obtained contains a large amount of salts, the removal of these salts is significantly important in the preparation of L-rhamnose. In the present invention, an effective process therefor is combined with a conventional method by using an ion exchange resin to thereby establish an efficient desalting process.