  We put the upper bound on the gravitational waves (GWs) induced by the
scalar-field fluctuations during the inflation. In particular, we focus on the
case where the scalar fluctuations get amplified within some subhorizon scales
by some mechanism during the inflation. Since the energy conservation law leads
to the upper bound on the energy density of the scalar fluctuations, the
amplitudes of the scalar fluctuations are constrained and therefore the induced
GWs are also. Taking into account this, we derive the upper bound on the
induced GWs. As a result, we find that the GW power spectrum must be $\mathcal
P_h \lesssim \mathcal O(\epsilon^2 (k/k_*)^2)$ up to the logarithmic factor,
where $\epsilon$ is the slow-roll parameter and $k_*$ is the peak scale of the
scalar-field fluctuations.
