A fishing-reel driving device includes a reel body having a shell for accommodating a reel driving mechanism. A holding frame includes an axially extending tubular mount with a coupled surface, and a radially extending web terminating at an anchoring edge to be secured to an anchored edge of the shell. A left side cover includes a peripheral abutment edge to be sleeved on the tubular mount, and a faceplate region extending radially to terminate at an outskirt edge. A tubular tightening member has a tubular coupling surface matingly engageable with the coupled surface, and a surrounding abutment edge to force the peripheral abutment edge to move towards the web. An actuating member is insertable into the shell to be coupled to the reel driving mechanism.