To enable to simulate while taking in the short time and transient variation relating with the simulating diffusion of exhaust gas on a road.

The device for simulating diffusion of exhaust gas on the road 1 for simulating the diffusion of exhaust gas on the road is constituted so as to obtain the concentration of the exhaust gas by a detail calculation model using a diffusion parameter and a particle diameter parameter, and includes a parameter setting part 8 for setting a diffusion parameter and an particle diameter parameter. The parameter setting part performs: a process for obtaining the concentration of the exhaust gas using the detail calculation model using a temporary diffusion parameter and temporary particle diameter parameter; a process for obtaining difference of concentration of the exhaust gas respectively obtained by the detail calculation model and experimental diffusion formula; a process for determining the difference by a determination value; and a process for setting the temporarily diffusion parameter and the temporarily particle diameter parameter as the diffusion parameter and the particle diameter parameter, under the condition that the difference is less than or equal to the determination value.