  In this paper we propose a mechanism of natural preheating of our universe by
introducing an inflaton field dependent mass term for the gravitational wave
for a specific class of massive gravity theory. For any single field
inflationary model, the inflaton must go through the oscillatory phase after
the end of inflation. As has recently been pointed out, if the gravitational
fluctuation has inflaton dependent mass term, there will be a resonant
amplification of the amplitude of the gravitational wave during the oscillatory
phase of inflaton. Because of this large enhancement of the amplitude of the
gravitational wave due to parametric resonance, we show that universe can
naturally go through the pre-reheated phase with minimally coupled matter
field. Therefore, during the reheating phase, there is no need to introduce any
arbitrary coupling between the matter field and the inflaton .
