The present invention provides a method for diagnosing and determining prognosis of gastric cancer in a subject by detecting suppressed expression of the REC8 gene, which in some cases is due to elevated methylation level in the genomic sequence of this gene. A kit and device useful for such a method are also provided. In addition, the present invention provides a method for treating gastric cancer by increasing REC8 gene expression or activity. Lastly, a highly sensitive and accurate detection method is provided for rapid determination of REC8 gene methylation status.