  Efforts to find the magnetic monopole in real space have been made in cosmic
rays and in accelerators, but up to now there is no firm evidence for its
existence due to the very heavy mass $\sim 10^{16}$GeV. However, we show that
the magnetic monopole can appear in the crystal-momentum space of solids in the
accessible low energy region ($\sim0.1-1$eV) in the context of the anomalous
Hall effect. We report experimental results together with first-principles
calculations on the ferromagnetic crystal SrRuO$_3$ that provide evidence for
the magnetic monopole in the crystal-momentum space.
