We report a combined study for the electronic structures of ferromagnetic CeAgSb$_2$ using soft X-ray absorption (XAS), magnetic circular dichroism (XMCD), and angle-resolved photoemission (ARPES) spectroscopies. The Ce $M_{4, 5}$ XAS spectra show very small satellite structures, reflecting a strongly localized character of the Ce $4f$ electrons. The linear dichroism effects in the Ce $M_{4, 5}$ XAS spectra demonstrate the ground state Ce $4f$ symmetry of $Gamma{_6}$, the spatial distribution of which is directed along the $c$-axis. The XMCD results give support to the picture of local-moment magnetism in CeAgSb$_2$. Moreover it is also found that the theoretical band dispersions for LaAgSb$_2$ provides better description of the ARPES band structures than those for CeAgSb$_2$. Nevertheless, ARPES spectra at the Ce $3d$-$4f$ resonance show the momentum dependence for the intensity ratio between Ce $4f^{1}_{5/2}$ and $4f^{1}_{7/2}$ peaks in a part of the Brillouin zone, suggesting the non-negligible momentum dependent hybridization effect between the Ce $4f$ and the conduction electrons. This is associated with the moderate mass enhancement in CeAgSb$_2$.
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