  We consider quadratic fluctuations around the tachyon vacuum numerically in
open string field theory. We work on a space ${\cal H}_N^{{\rm vac}}$ spanned
by basis string states used in the Schnabl's vacuum solution. We show that the
truncated form of the Schnabl's vacuum solution on ${\cal H}_N^{{\rm vac}}$ is
well-behaved in numerical work. The orthogonal basis for the new BRST operator
$\tilde Q$ on ${\cal H}_N^{{\rm vac}}$ and the quadratic forms of potentials
for independent fields around the vacuum are obtained. Our numerical results
support that the Schnabl's vacuum solution represents the minimum energy
solution for arbitrary fluctuations also in open string field theory.
