  We discuss gravitational waves (GWs) induced by a heavy spectator field that
starts to oscillate during inflation. During the oscillation of the spectator
field, its effective mass can also oscillate in some potentials. This mass
oscillation can resonantly amplify the spectator field fluctuations. We show
that these amplified fluctuations can induce large GWs, which could be
investigated by future gravitational wave observations. This kind of induced GW
can be produced even if the spectator field does not have any interaction with
other fields except for gravitational interaction.
