  We discuss the scalar mode of gravitational waves emerging in the context of
$F(R)$ gravity by taking into account the chameleon mechanism. Assuming a toy
model with a specific matter distribution to reproduce the environment of
detection experiment by a ground-based gravitational wave observatory, we find
that chameleon mechanism remarkably suppresses the scalar wave in the
atmosphere of Earth, compared with the tensor modes of the gravitational waves.
We also discuss the possibility to detect and constrain scalar waves by the
current gravitational observatories and advocate a necessity of the future
space-based observations.
