The Japanese Foundation for Cancer Research (JFCR) has provided a large database on gastric cancer for open use comprised of data on survival times after surgery and related covariates including prognostic factors. Data analysis based on the generalized hazards model incorporating the B-spline function (GHMBS) was performed using a dataset (JFCR dataset) composed of 9,631 cases within this database. A model selection method was adopted for clarifying the meaning of estimated parameters, because the GHMBS was comprised of the proportional hazards model (PHM) and accelerated failure time model (AFTM) as submodels. A preliminary simulation experiment to examine the performance of the model selection method based on the GHMBS was conducted under the condition that multiple covariates were considered, where one was the target covariate and the other was covariate for adjustment. After validation of the method by this simulation experiment, the method was applied to the JFCR dataset to estimate the period effect for prolonging survival time with an adjustment for the stage effect. The analysis revealed that the PHM was suitable for the period effect, while a mixture of the PHM and AFTM was for the stage effect and treatment for gastric cancer made steady progress from the 1950s through the 1990s.