The ALPACA experiment, which consists of a large air shower (AS) array and water-Cherenkov-type muon detectors, is a new project to observe sub-PeV gamma rays in the southern hemisphere. The prototype AS array, named ALPAQUITA ($1/4$ scale of ALPACA), has been prior operated in the center of the ALPACA site at Chacaltaya plateau (4,740 m a.s.l.) in Bolivia. The ALPAQUITA AS array consists of 97 scintillation detectors each with an area of 1 m$^2$, deployed with 15 m spacing. Using this AS array, we have successfully observed the cosmic-ray Moon shadow with a statistical significance $-6.2\sigma$ for 73 live days. The angular resolution is estimated to be 0.9$^{\circ}$ based on the Moon shadow observation and the even-odd method. In this proceedings, we assess the performance of ALPAQUITA AS array, and outline the future prospects of the sub-PeV gamma-ray observation with the ALPAQUITA AS+MD array.