  We investigate the effect of isotope substitution on the electron-phonon
interaction in the multi-band superconductor MgB2 using tunable laser based
Angle Resolved Photoemission Spectroscopy. The kink structure around 70 meV in
the $\sigma$ band, which is caused by electron coupling to the E2g phonon mode,
is shifted to higher binding energy by ~3.5 meV in Mg(10)B2 and the shift is
not affected by superconducting transition. These results serve as the
benchmark for investigations of isotope effects in known, unconventional
superconductors and newly discovered superconductors where the origin of
pairing is unknown.
