This paper presents features for quantitatively diagnosing the progression of gastric atrophy from stomach X-ray images, aiming to support mass screening for gastric cancer. Specifically, the features focus on the gastric areae shadow pattern, which appears as a mesh-like and low-contrast shadow that becomes prominent in cases of moderate to severe atrophy. In moderate atrophy, this pattern appears in the lower stomach region and gradually extends upward as the condition progresses. To capture this trend, the stomach area is divided into upper and lower regions, and the features are extracted by measuring the area occupied by the gastric areae shadow pattern in each region. An evaluation conducted on 157 gastric X-ray images demonstrated that the proposed features increased in correlation with atrophy progression and could effectively distinguish between normal and abnormal cases without reliance on specific classifiers.