  We present a theory of the isotope effect of the Curie temperature $T_{\rm
c}$ in itinerant ferromagnets. The isotope effect in ferromagnets occurs via
the electron-phonon vertex correction and the effective attractive interaction
mediated by the electron-phonon interaction. The decrease of the Debye
frequency increases the relative strength of the Coulomb interaction, which
results in a positive isotope shift of $T_{\rm c}$ when the mass $M$ of an atom
increases. Following this picture, we evaluate the isotope effect of $T_{\rm
c}$ by using the Stoner theory and a spin-fluctuation theory. When $T_{\rm c}$
is large enough as large as or more than 100K, the isotope effect on $T_{\rm
c}$ can be measurable. Recently, precise measurements on the oxygen isotope
effect on $T_{\rm c}$ have been performed for itinerant ferromagnet SrRuO$_3$
with $T_{\rm c}\sim 160$K. A clear isotope effect has been observed with the
positive shift of $T_{\rm c}\sim 1$K by isotope substitution
($^{16}O\rightarrow ^{18}O$). This experimental result is consistent with our
theory.
