A gear shifting device of a drum fishing reel includes first and second gears having first and second through holes respectively, and a gear shaft extending through the first and second through holes and having two through grooves. A shifting rod is provided with two elongated holes and axially moveably inserted into a shaft hole of the gear shaft. A key is inserted through the through grooves and the elongated holes and abutted with a spring disposed inside the shifting rod. When the key is inserted into the first through hole or the second through hole, the first gear or the second gear is rotatably driven by the gear shaft. As such, the key can be positioned positively, the speed changing motion acts smoothly, and the gear shifting device can be easily assembled with stable quality.
