A cover device for a storage battery includes a sub-cover that includes a flat bottom with a plurality of first vent-holes, a top portion and a connecting wall which interconnects the flat bottom and the top portion to define a space therein. The cover device further includes a plurality of tubes, each of which having n open end which is connected to each of the first vent-holes of the flat bottom and a closed end which is plugged into a respective inlet-hole of a main cover of the storage battery so as to close the inlet-hole, and a second vent-hole which is formed in the sub-cover and which communicates the space in the sub-cover with an exterior of the same.