This paper presents a method of semi-supervised learning based on tri-training for gastritis classification using gastric X-ray images. The proposed method is constructed based on the tri-training architecture, and the strategies of label smoothing regularization and random erasing augmentation are utilized in the method to enhance the performance. Although the task of gastritis classification is challenging, we report that the proposed semi-supervised learning method using only a small number of labeled data achieves 0.888 harmonic mean of sensitivity and specificity on test data composed of 615 patients.