Earthquake ground motions have been observed at Haneda Airport for more than 14 years.The Port and Airport Research Institute have been in charge of collection and analyses of the data.Through the analyses of the data, the authors would like to retrieve a wide range of technical information which would play a vital role in the future construction of facilities not only at this particular airport but also at other airports which are located on soft soil layers. In the present technical report, the authors evaluated the site amplification factors at Haneda Airport based on spectral inversion technique. The data set comprises 1384 records (2768 horizontal components) from 31 earthquakes recorded at 122 stations in and around the Kanto plain, Japan, including not only stations at Haneda Airport but also K-NET stations and stations for strong-motion observation in Japanese Ports. Two cases of inversion analysis were carried out. For the first case, the quality factor Q was determined through the inversion analysis. For the second case, Q was prescribed based on Kinoshita's (1994) results. The K-NET station at Hinohara (TKY002) was selected as a reference site.The two inversion cases led to almost identical results as long as the site amplification factors are concerned. Although the site amplification factors at sites No.1 through No.8 at Haneda Airport are almost identical at frequencies lower than 0.2Hz, the site ampliflCation factors are quite location-dependent at frequencies higher than 0.2Hz. At sites No.1,2,3,4 and 7, where the soft soil layers are thick, the site amplification factors are larger than 10 at frequencies 0.3-1.OHz. At sites No.5,6 and 8, where the soft soil layers are thin, the peaks of site amplification factors are located at higher frequencies. The obtained site amplification factors can be used for strong motion prediction in conjunction with the stochastic Green's function method.