Provided a side cover detachable apparatus of a dual bearing reel for fishing. The apparatus includes a boss, a rod, and a spring. The boss is provided within a side cover, has a stop hole at a center, and has a slope surface around a front end. The rod moves to the left/right with passing through a reel frame and the side cover, and is selectively inserted into the stop hole of the boss. The spring is fitted around the rod and elastically supports left/right movement of the rod.