A fastening device includes a case, a spool, a locking unit and a knob. The case includes a receiving space and a plurality of side teeth. The spool is disposed within the case, and the lace is wound therearound. The locking unit includes a ring body and at least one pawl arm. The ring body is optionally coupled to the spool and includes a blocking portion. The at least one pawl arm includes a first terminal and a second terminal. The second terminal is engaged with the side teeth in a loosening direction and is disengaged from the side teeth in a tightening direction. The second terminal includes an abutting portion abutted against the blocking portion in the loosening direction.