NOVELTY - A genetically-engineered algae capable of secreting crystalline cellulose, is new. The engineered algae comprises blue cesA gene comprising a 2387 base pairs (SEQ ID NO: 1) fully defined in the specification, and mutant Synechococcus species (PCC 7002).    USE - The genetically engineered algae is useful for producing cellulose, which is used for producing film material, artificial skin, food additive and ethanol for biofuel (all claimed).    ADVANTAGE - The produced cellulose has excellent crystallinity. The produced film material has high quality.    DETAILED DESCRIPTION - A genetically-engineered algae capable of secreting crystalline cellulose, is new. The engineered algae comprises blue cesA gene comprising a 2387 base pairs (SEQ ID NO: 1) fully defined in the specification, and mutant Synechococcus species (PCC 7002). The genome of the mutant capable of integrating and expressing Gluconacetobacter xylinum (ATCC 53582) comprising a 7 continuous cellulose gene cluster relative to violacein synthesis. The gene cluster comprises a cmcase comprising a 1029 base pair (SEQ ID NO: 4) fully defined in the specification, ccp gene comprising a 1062 base pair (SEQ ID NO: 5) fully defined in the specification, acsA gene comprising a 2172 base pair (SEQ ID NO: 6) fully defined in the specification, acsB gene comprising a 2727 base pair (SEQ ID NO: 7) fully defined in the specification, acsC gene comprising a 3909 base pair (SEQ ID NO: 8) fully defined in the specification, acsD gene comprising a 471 base pair sequence (SEQ ID NO: 9) fully defined in the specification, bglxA gene comprising a 3042 base pair sequence (SEQ ID NO: 10) fully defined in the specification, CG27 gene fragment comprising a 100392 base pair (SEQ ID NO: 12) fully defined in the specification as starting sequence, and termination gene comprising a 1000 base pair (SEQ ID NO: 13) fully defined in the specification. INDEPENDENT CLAIMS are included for the following: (1) method (M1) of preparing the genetically-engineered algae, involves (a) deleting damaged or mutant cellulose gene cesA of SEQ ID NO: 1 of Synechococcus sp., (b) constructing BAC vector by connecting the erythrocin resistance gene (Em) on the carrier, and on one segment having size of 1.2 kb of Synechococcus sp. homologous to genome comprising a 1209 base pair (SEQ ID NO: 11) fully defined in the specification, (c) inserting the above-mentioned cellulose gene cluster relative to violacein synthesis to the BAC vector, and (d) inserting the obtained BAC vector into mutant blue-green algae; and (2) method (M2) for producing crystalline cellulose, involves (a) preparing genetically engineered blue-green algae, (b) culturing obtained algae in culture medium for 1 week, and terminating the culturing process, when the optical density (OD) value of the blue algae reaches to 1-2, (c) concentrating and centrifuging the algae engineering cultured algae, transferring the centrifuged product to the fresh culture medium which is suitable for growth of the algae of BGll for 2 weeks, and (d) extracting the fiber from the algae.