  Gamma-Ray Bursts (GRBs) are now considered as relativistic jets. We analyze
the gravitational waves from the acceleration stage of the GRB jets. We show
that (i) the point mass approximation is not appropriate if the opening
half-angle of the jet is larger than the inverse of the Lorentz factor of the
jet, (ii) the gravitational waveform has many step function like jumps, and
  (iii) the practical DECIGO and BBO may detect such an event if the GRBs occur
in Local group of galaxy. We found that the light curve of GRBs and the
gravitational waveform are anti-correlated so that the detection of the
gravitational wave is indispensable to determine the structure of GRB jets.
