  The sensitivity to the sterile neutrino mixing at very short baseline reactor
neutrino experiments is investigated. In the case of conventional (thermal
neutron) reactors it is found that the sensitivity is lost for $\Delta m^2
\gtrsim$ 1 eV$^2$ due to smearing of the reactor core size. On the other hand,
in the case of an experimental fast neutron reactor Joyo, because of its small
size, sensitivity to $\sin^22\theta_{14}$ can be as good as 0.03 for $\Delta
m^2 \sim$ several eV$^2$ with the Bugey-like detector setup.
