Abnormally high temperature inside the long, undergrounded tunnel is a problem in the urban area. As one of the countermeasures, a ventilation fan has been operated. However, the insufficient temperature reduction effect and high cost are an issue and improvement is required. In this study, in order to improve the operating plan of ventilation, the flow field and temperature distribution characteristics are clarified by computational thermal fluid analysis, and the operation of the optimum ventilation was decided. The dominant factors for the temperature rising were identified as traffic volume, lane axis wind speed, cross flow ventilation, and heat flux between tunnel body and air in tunnel. In the analysis, we focused on these four factors, and applied these factors obtained from long-term on-site measurement to the boundary condition and the initial condition. In addition, the amount of heat from vehicle traffic was calculated based on the measurement and the past report results. The analytical model is 1000 m partial tunnel section where the temperature rising was intense. The validation of numerical model was verified from the comparison between the analysis results and the measured values. It was confirmed that the effect of increasing lane axis wind speed as a countermeasure was not significant, and the ventilation amount of crosswind is recommended as 60 m3/s.