observation of mid-infrared emission lines in low-metallicity star-forming galaxies. Thus, we change the SED of ISRF, and adopt the following fitting formula according to the averaged SED of the BCD sample in @boselli03: $$\begin{aligned}
4\pi\lambda J_\lambda^{(0)} =\left\{
\begin{array}{l}
0 ~~~~~ (\lambda_{\mu\mathrm{m}}\leq 0.0912), \\
3.02\times 10^{-4}\lambda_{\mu\mathrm{m}}^{-1.20}\\
~~~~~~~(0.0912<\lambda_{\mu\mathrm{m}}\leq 0.25), \\
1.55\times 10^{-3}\times 10^{-3}
\lambda_{\mu\mathrm{m}}^{-0.0371}\\ ~~~~~~~
(0.25<\lambda_{\mu\mathrm{m}}\leq 0.37), \\
2.39\lambda_{\mu\mathrm{m}}^{7.41}~~~~~
%~~~~~~~
(0.37<\lambda_{\mu\mathrm{m}}\leq 0.4), \\
4\pi\lambda [W_1B_\lambda (T_1)+W_2B_\lambda (T_2)]
\\ ~~~~~~~ 
(0.4<\lambda_{\mu\mathrm{m}}),
\end{array}
\right.\label{eq:isrf}\end{aligned}$$ where $J_\lambda^{(0)}$ is the ISRF intensity whose normalization is determined so that the intensity at wavelength $\lambda =0.2~\mu$m (almost the centre of the UV range; @buat96) is equal to the solar neighborhood ISRF estimated by @mathis83 ($4\pi\lambda J_\lambda^{(0)}$ is expressed in units of erg cm$^{-2}$ s$^{-1}$), $\lambda_{\mu\mathrm{m}}$ is the wavelength in units of $\mu$m, $B_\lambda (T)$ is the Planck function, $(T_1,\, T_2)=(1000,\, 4000~\mathrm{K})$, and $(W_1,\, W_2)=(1.61\times 10^{-15},\, 2.90\times 10^{-14})$.

In this paper, we newly include the effect of dust extinction on the ISRF. We simply assume that the ISRF intensity, $J_\lambda$, at optical depth $\tau_\lambda$ is $$\begin{aligned}
J_\lambda=J_\lambda^{(0)}\chi\exp(-\tau_\lambda)\, ,\end{aligned}$$ where $\chi$ is the scaling factor of the ISRF relative to $J_\lambda^{(0)}$ and is assumed to be independent of wavelength. For the wavelength dependence of $\tau_\lambda$, we assume the Milky Way 