[Purpose] This study aimed to examine the validity and reproducibility of a new quantitative method for measuring spinal kyphosis using computed tomography (CT), and to investigate its relationship with reflux esophagitis. [Method] Using a new method to measure the index of kyphosis in CT images (IKCT), 10 examiners evaluated 10 cases of spinal kyphosis. One examiner measured 47 cases twice and 20 cases were examined to assess the validity with the kyphosis index. A case-control study was conducted on 303 cases of reflux esophagitis, of which 241 were mild and 62 severe. [Results] Regarding IKCT reproducibility, the inter-rater intraclass correlation coefficient was 0.977. The intra-rater intraclass correlation coefficient was 0.974. The correlation index with the kyphosis index was 0.731. A greater IKCT value, not contracting serious atrophic gastritis, and severe hiatal hernia were identified as risk factors for severe reflux esophagitis. [Conclusion] IKCT is a simple and useful method for measuring kyphosis. The prevention of kyphosis can help suppress severe reflux esophagitis.