In Japan, while there has not yet been a confirmed increase in dengue fever incidence, the vector mosquito, Aedes albopictus, is widely distributed, indicating the potential for more frequent outbreaks in the future. In this study, we develop a mathematical model by constructing the reproduction number from a bottom-up approach, allowing us to evaluate the long-term risk of dengue. Although the model is relatively simple, relying on temperature as the predictive variable, it has proven effective in estimating the likelihood of transmission and epidemics. Even in relatively optimistic scenarios, the inter-epidemic period—during which there is no risk of dengue fever outbreaks—was reduced by approximately 20 days in Tokyo. Geographically, the regions at risk for dengue are expected to shift northward, and in more pessimistic scenarios, the risk could extend as far as Hokkaido. The mosquito biting rate was found to influence the reproduction number in a quadratic manner, mathematically reinforcing the importance of focusing efforts on mosquito bite prevention. Considering future regional adaptation strategies is essential.