  We report the measurement of one-body loss rates for magnetically trapped
metastable europium atoms and the study of their loss mechanism. The loss of
atoms observed in a magneto-optical trap is not fully understood because of the
indivisibility of the loss regarding optical pumpings due to the presence of
cooling laser beams. We magnetically trap the atoms by directly loading from
the magneto-optical trap and observe almost identical one-body loss rates of
approximately $2.6\,\mathrm{s^{-1}}$ for two isotopes: $\mathrm{{}^{151}Eu}$
and $\mathrm{{}^{153}Eu}$. Our rate-equation-based model, combined with loss
rate measurements carried out with repumpers, shows that blackbody radiation at
room temperature drives E1 transitions and induces the observed losses.
