We present an analytical result on an outstanding gravity wave motion observed with the MU radar at Shigaraki (34.9-degrees-N, 136.1-degrees-E), Japan, on 5 September 1988. In the analysis the three components of wind velocity, estimated from the Doppler shift of the radar echo backscattered in the mesosphere, are used. The velocity data set shows that a long-period wave motion is dominant only in the horizontal component while short-period wave motions are commonly seen in the vertical and horizontal components. The large amplitude ratio between the horizontal and vertical components of the long-period wave motion seems to be due to the propagation property of an inertio-gravity wave. Our data also show that the wave amplitude is saturated at altitudes higher than 75 km while it is kept anomalously large (approximately 50 m s-1). The wave parameters estimated from our hodograph analysis lead to the conclusion that the behaviour of the wave is not inconsistent with the linear saturation theory for monochromatic inertio-gravity waves.