Hα-to-FUV total flux ratios predicted by our models. The black curves shows values predicted by models with different IMF slopes (α) and truncation masses (M_u) for the set of steep models and the set of truncated models, respectively. The first dot from the left is from the model with the standard Salpeter IMF (α = 2.35, M_u = 100 M_⊙). The horizontal red line indicates the observed value in the M 83 outer disk. The shaded area around the red line shows the error in the observed value. The change in the observed flux ratio if is assumed to be larger by 0.1 mag is shown with a black arrow. The observations and models agree for 3.0 < α < 3.1 or 30 M_⊙ < M_u < 40 M_⊙.