In the family of narrow-gap semiconductors, iron monosilicide is famous for its unusual properties [1]. For instance, the violation of sum rules in FeSi has attracted wide attention in optical spectroscopy, and metallization with increasing temperature occurs much faster than it could be expected from band-structure calculations. Near the Fermi level in the energy spectrum of FeSi, there lies a narrow threefold degenerate band formed by the d-electrons of iron. Various theoretical models in which the strong electron correlations of the d-electrons are taken into account have been used to explain the observed anomalies [1-3].