The MU radar (34.9¿N, 136.1¿E, geomagnetic latitude 25.0¿N) is utilized to observe the spatial structures of the field-aligned irregularities in the midlatitude E region. The multibeam observation in 12 directions has found that the ''quasi-periodic'' echoes have a monochromatic wave structure propagating toward SSW. Much smaller-scale structure of the irregularities has been observed by the interferometry techniques with three receivers. The results are consistent with a model in which a southward propagating gravity wave modulates the sporadic E layer to generate the quasi-periodic echoes of the irregularities.