In this study, we analyzed the long‐term reanalysis data set JRA‐55 using the WRF regional meteorological model for the heavy rain that caused a large‐scale landslide disaster in the upper Tenryu River basin in June 1961. The heavy rainfall of June 1961 was reproduced with a temporal resolution of 1 hour and a horizontal resolution of 1 km. The simulation results were analyzed by comparing the calculated results with the ground rain gauge observations. This is the first attempt in the literature to analyze the cause of heavy rain in the era when there were no radar rain gauges. The detailed rainfall distribution of the upper Tenryu River basin has been reproduced. From the reproduction results, we found the cause of the torrential rain. It was found that the cause of the heavy rains in areas where landslides were severe was convergence field to the strong influence of independent peaks and narrow topography in the upper reaches of the Tenryu River, and convergence field strengthened rainfall. This work was derived from the WRF model using universal equations. For this reason, it is possible that the results obtained in this research are also valid for other torrential rains. It is expected that the results of this research will be useful for future disaster prevention and erosion control plans.