Simultaneous observations of short-period gravity waves were carried out using three all-sky cooled-CCD imagers of OH airglow at Moshiri (northern edge of Japan) and at Shigaraki and Bisei (middle part of Japan) during the FRONT campaign of May 1998. These stations were separated by horizontal distances of 250--1300 km, and they offered a unique opportunity to investigate the spatial extent of gravity waves in the mesopause region. The images from Shigaraki and Bisei showed short-period gravity waves (horizontal wavelength: 20--40 km, phase front: ~east-west aligned) moving northward (phase velocity: 20--40 m/s) on May 19, 21 and 22. At Moshiri, similar east-west waves moving northward were observed on May 19 and 21, indicating a broad horizontal extent of the gravity waves. Waves at Moshiri were not evident on May 22, however. The horizontal wind measured by the MF radar at Wakkanai (near Moshiri) showed height profiles that were similar to those at Yamagawa (southern edge of Japan) on May 19 and 21 but quite dissimilar on May 22. On the basis of these data, we suggest that short-period gravity waves in the mesopause region can have a broad spatial extent of more than 1000 km. The generation and propagation of internal gravity waves are discussed as a cause of the spatial extent of the waves. ¿ 2000 American Geophysical Union