to the coherent magnetic field strength. In this limit, the timescale $t_{\rm diff}$ for the electron/positron trapping in the shocked region is given by $$t_{\rm diff}=\frac{d^2}{2D_{\rm sh}}=\frac{3}{2}\frac{eBd^2}{\varepsilon_ec},$$ where $D_{\rm sh}=cr_{\rm g}/3$ is the diffusion coefficient under the Bohm limit, $d$ is the size of the shocked region and $r_g=\varepsilon_e/eB$ is the Larmor radius of electron/positron with energy $\varepsilon_e$. To estimate the diffusion timescale, we need to know the size and the magnetic field strength of the shocked region. Here we consider the time $t>t_{\rm dec}$, so that the size of the shocked region is an order of the radius of forward shock front at $t=t_{\rm dec}$, $d\sim R_{\rm out}(t=t_{\rm dec})\sim 3\times 10^{19}{\rm cm}$. For the radius of the inner shock front at $t>t_{\rm dec}$, we use the balance condition between the momentum transferred by wind and the pressure of ISM $$\frac{L_{\rm sd}}{4\pi R^2_{\rm in}c}=p.$$ For the fiducial parameters, $R_{\rm in}(t>t_{\rm dec})\sim 2\times 10^{17}$ cm. The strength of the magnetic field at the inner radius $B_{\rm in}$ can be estimated as $B_{\rm in}\sim 2 \times 10^{-6}$ G, which is almost the same as that of ISM. Then, the diffusion timescale is $$t_{\rm diff}\sim 2 \times 10^4 \left(\frac{\varepsilon_e}{50 {\rm TeV}}\right)^{-1} {\rm yr}.$$ 