Experiments with high angular resolution that can detect gamma rays in the 100 TeV region will help study acceleration mechanisms of gamma-ray sources with morphological characteristics. In recent years, three groups using extensive air-shower (AS) arrays have reported detecting gamma rays in the 100 TeV region from celestial bodies in the galaxy. In extensive air shower experiments, an arrival direction of a cosmic ray is determined by estimating a shape of a front surface of an air shower based on the density distribution and detection time of secondary particles. The density of secondary gamma rays of an air shower is several times that of secondary electrons and positrons in an air shower and the difference increases as the distance from the shower axis increases. Therefore, increasing the sensitivity to detect secondary gamma rays plays an important role in determining the arrival direction of the shower. Monte Carlo simulation was used to investigate the effect of secondary gamma rays on the time determination accuracy of the air shower front. When an AS array capable of measuring secondary gamma rays with 100% sensitivity was installed at an altitude of 4,740 m, the angular resolution for a 500 TeV shower was improved by about 40% compared to an array that could observe only secondary electrons and positrons. The optimum shape of the water Cherenkov detector (WCD), which is highly sensitive to gamma rays, was investigated by Monte Carlo simulation. When a water tank with a radius of 4.5 m and PMTs with a diameter of 8 inches was used, it was found that about 19 PMTs (area ratio of approximately 0.01) were suitable for detection efficiency and timing response at a water depth of 1.6 m.