To explore the southern gamma-ray sky in the sub-PeV region ALPACA is now under construction near the Chacaltaya mountain in Bolivia. ALPACA is expected to discover various sub-PeV emitters, which are accelerating hadrons beyond PeV energy. To determine the maximum acceleration energy in the galaxy, a square kilo meter (a Mega square meters) air shower array, Mega ALPACA is discussed as a next generation observatory. With high-statistics observations not only the highest energy object, but also distribution of diffuse gamma rays, nearby active galaxies and space weather are also in the main scope of Mega ALPACA. Design and possible scientific potentials of Mega ALPACA are presented.