Plasmodium vivax re-emerged in 1993 near the demilitarized zone (DMZ) in South Korea, although P. vivax malaria disappeared in South Korea in 1979. The re-emergence of malaria in South Korea is believed to have originated from infection by mosquitoes from North Korea across the DMZ. The principal vector of P. vivax in the Korean Peninsula is Anopheles sinensis. The density of An. sinensis has a peak during the second week of July. The North Korean strain of P. vivax has 2 characteristics: a wide distribution of the terms of relapse and a high rate of relapse. Therefore, we may well wonder why the incidence of malaria is concentrated in summer, especially in August. Mathematical models in North Korea and South Korea were constructed in which the South Korean model was affected unidirectionally by the North Korean model. We carried out simulations of the model for the Paju-shi and Yonchon-gun situations near the DMZ region. The simulation results followed the time-course of the re-emergence of P. vivax there, and revealed the mechanism of the elevation of the incidence of P. vivax in summer.