From an analysis of dense pibal data in terms of time and space, it is shown that a nocturnal low-level jet exists on summer days over the Kanto plains, Japan. This will be the first presentation of an analytical study on the nocturnal low-level jet flowing over Japan.
   The low-level jet studied in this report is quite similar to that over the Great Plains of the United States, though it is smaller in horizontal scale. Southwest jet maxima, about 20 m s^-1, occur around midnight at an altitude of 200 to 700 m. Observed wind maxima seem to be supergeostrophic in terms of large-scale pressure field. An interesting result is that as the jet flow passes over land, the amplitude of diurnal wind variations of wind speeds becomes greater with decreasing average wind speeds, the height of the maximum amplitude becomes lower, and the momentum of the jet concentrates in the relatively lower layer. A discussion referring to momentum concentration in the lower layer with time is made between the low-level jet and the nocturnal cyclonic vortex both found in the Kanto Plains.