We present the first wide range (2 meV−5.5 eV) optical conductivity of EuB6 from reflectivity and ellipsometry measurements. Upon the ferromagnetic transition at Tc=15.5 K, interband transition σ1(ω) decreases at three different energy ω=0.5, 1.4, 2.75 eV and the lost spectral weight is transferred to the Drude σ1(ω) at ω <0.33 eV. We succeeded in explaining this unprecedented multiple-energy σ1(ω) change using LDA+U calculated band structure by Kunes and Pickett (Phys. Rev. B, 69, 165111). Our finding supports strongly that (1) EuB6 is a semimetal and (2) the exchange-driven band splitting is the primary source of the drastic ρ(T ) and ω 2 p changes at T < Tc.PACS numbers: 78.20. Ls,78.40.Kc,71.20.Eh,71.70.Gm Interplay of itinerant carrier with localized magnetic moment can lead to novel magneto-resistance effect in solid. When the moments align with long range ferromagnetic order, dc-resistivity exhibits a drastic change as seen in EuO and (Ga,Mn)As. Theories such as the Kondo-lattice model, Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction and spin-polaron model propose possible microscopic mechanism of the carrier-moment coupling in magnetic conductors.Europium hexaboride EuB 6 is a simple cubic compound where dilute conducting carrier (n ∼ 10 19 cm −3 ) coexists with localized 4f spin of Eu +2 (S=7/2). The latter undergoes ferromagnetic (FM) transition at T c =15.5 K where simultaneously dc-resistivity ρ drops sharply [1] and the infrared plasma frequency ω 2 p (=4πe 2 n/m) suddenly increases [2]. Such dramatic FM-driven changes were seen in the manganites like (La 0.7 Ca 0.3 )MnO 3 [3] whereas EuB 6 being free from complications like lattice distortion and random substitution offers a cleaner and ideal example of ferromagnetic metal where the carrier-moment interaction can be studied in more fundamental level.In the Kondo-lattice model itinerant electron band is split in FM phase by the s-f exchange interaction into two spin-polarized sub-bands. Kreissl and Nolting applied this theory to EuB 6 to find that the band splitting brings about a significant increase of the carrier density [4]. The ρ(T ) drop and ω 2 p increase are attributed to it. In contrast Hirsch proposed an alternative model that conduction band becomes broader in FM state because the spin-polarization reduces the bond-charge repulsion effect [5,6]. This band width increase or equivalently carrier mass decrease is considered as a source of the enhanced metallicity. Pereira et al. suggested yet another scenario that at T>T c the disordered spin background can localize the carriers via their exchange interaction and it introduces a mobility edge in the conduction band [7]. FM spin alignment reduces the localization effect, lowering the mobility edge position, and the carriers become delocalized. This theoretical diversity shows that EuB 6 is an important prototype of metallic ferromagnetism. At present it is not clear which model is correctly describing it.Probing electronic band structure is an essential step to solve this issue. Accordin...