A stopper for plugging a developer filling port includes a stopper main body and a lock member engageable with the stopper main body. The stopper main body is configured to be fitted to the filling port and shaped like a tube with a bottom. The lock member has a rigidity greater than that of the stopper main body, the lock member engaged with the stopper main body serves to restrict deformation of the stopper main body. A developing device for use in an image forming apparatus includes the stopper configured as described above to be fitted to its developer filling port.