The accurate pH range of peritoneal fluid is clinically valuable for the evaluation of some pathological conditions of the body, however, it is not easy to measure in healthy individuals. The aim of this study was to measure; pH, pCO₂, pO₂, Na⁺, K⁺⁺, Ca⁺⁺, HCO₃^-, and O₂ saturation of the peritoneal fluid in patients with non-serosal invasive gastric cancer. One hundred and thirty four patients (86 men and 48 women), ranging in age from 24 to 91 years were enrolled in this study. After opening the abdominal wall, the probe of a portable pH meter was placed in the peritoneal fluid in the subhepatic space. In addition, I collected the peritoneal fluid from the subhepatic space to measure, pH, pCO₂, pO₂, Na⁺, K⁺⁺, Ca⁺⁺, HCO3^-, and O₂ saturation using an autoanalyzer. The pHs of the peritoneal fluids tested has a mean of 7.73 (range 7.46 - 8.10), and the other parameters were pCO₂, 22.81 mmHg; pO₂, 136.49 mmHg; Na⁺, 146.57 mmol/L; K⁺⁺, 4.80 mmol/L; Ca⁺⁺, 0.89 mmol/L; HCO₃^-, 30.54 mmol/L, and O₂ saturation, 99.74%. This study describes a practical method of measuring the pH of peritoneal fluid. The result obtained reflects the normal adult peritoneal pH value, which I propose as a reference value.