A holder locking structure includes: a connector housing and a front holder. The connector housing includes: a fitting hood to which another connector is inserted and fit; a terminal accommodation accommodating a terminal fitting; a rib which is protruded into the fitting hood and is configured to prevent terminal connection fault of the terminal fitting; and a first locking portion configured to lock the front holder. The front holder is inserted in the fitting hood of the connector housing and is locked to the connector housing to prevent removal of the terminal fitting. The front holder is provided with a rib insertion hole to which the rib is inserted and a jig insertion hole into which a jig can be inserted to unlock the front holder from the connector housing. The rib insertion hole and the jig insertion hole are continuously provided.