Periodic bursts of very intense electrostatic electron cyclotron harmonic waves have been discovered by the PWP (Plasma Wave Spectra Probe) onboard the Sakigake spacecraft while the spacecraft was crossing the plasma sheet boundary layer within the distance range from 40 to 50 earth radii. The observed pulsive bursts of the plasma waves show very intense nature which is characterized by BEN (broadband electrostatic noise) with the maximum intensity of about 200 mV/m and the bursts are enhanced periodically with the period around 1.5 minutes suggesting large volume (within the inferred size from 100 km to about 1000 km) of plasma cloud periodically flowing toward tail region. There were also periodic variation of ESCH wave spectra suggesting the velocity space modulation in the plasma flow with period of ranging from 300 to 400 sec; the magnetic field intensity shows the periodic variation with the same characteristic periods from 300 to 400 sec.