  The temperature dependence of the electronic structure of CeBi arising from
two types of antiferromagnetic transitions based on optical conductivity
($\sigma(\omega)$) was observed. The $\sigma(\omega)$ spectrum continuously and
discontinuously changes at 25 and 11 K, respectively. Between these
temperatures, two peaks in the spectrum rapidly shift to the opposite energy
sides as the temperature changes. Through a comparison with the band
calculation as well as with the theoretical $\sigma(\omega)$ spectrum, this
peak shift was explained by the energy shift of the Bi $6p$ band due to the
mixing effect between the Ce $4f \Gamma_8$ and Bi $6p$ states. The single-layer
antiferromagnetic ($+-$) transition from the paramagnetic state was concluded
to be of the second order. The marked changes in the $\sigma(\omega)$ spectrum
at 11 K, however, indicated the change in the electronic structure was due to a
first-order-like magnetic transition from a single-layer to a double-layer
($++--$) antiferromagnetic phase.
