  We observe a magnetic Feshbach resonance in a collision between the ground
and metastable states of two-electron atoms of ytterbium (Yb). We measure the
on-site interaction of doubly-occupied sites of an atomic Mott insulator state
in a three-dimensional optical lattice as a collisional frequency shift in a
high-resolution laser spectroscopy. The observed spectra are well fitted by a
simple theoretical formula, in which two particles with an s-wave contact
interaction are confined in a harmonic trap. This analysis reveals a wide
variation of the interaction with a resonance behavior around a magnetic field
of about 1.1 Gauss for the energetically lowest magnetic sublevel of
${}^{170}$Yb, as well as around 360 mG for the energetically highest magnetic
sublevel of ${}^{174}$Yb. The observed Feshbach resonance can only be induced
by an anisotropic inter-atomic interaction. This novel scheme will open the
door to a variety of study using two-electron atoms with tunable interaction.
