  The low energy effective potential for the model with a light scalar and a
heavy scalar is derived. We perform the path integration for both heavy and
light scalars and derive the low energy effective potential in terms of only
the light scalar. The effective potential is independent of the renormalization
scale approximately. By setting the renormalization scale equal to the mass of
the heavy scalar, one finds the corrections with the logarithm of the ratio of
the two scalar masses. The large logarithm is summed with the renormalization
group (RG) and the RG improved effective potential is derived. The improved
effective potential includes the one-loop correction of the heavy scalar and
the leading logarithmic corrections due to the light scalar. We study the
correction to the vacuum expectation value of the light scalar and the
dependence on the mass of the heavy scalar.
