Abstract: In this paper by use of space-time spectral anaysis the anomaly of monthly average 500hPa height along the latitude 20°N for Jan. 1951-Dec. 1980 is analyzed and statistical character of ultra-long wave in the low latitude is discussed.1). According to equation (8), the zonal mean of auto-correlation R_k(τ) at time lag τ(month) is computed. The result shown in Fig 1. The decrease of R_k(τ) with the increase of τ for k=0.3 is slower than k =5. This means that the persistence of ultra-long wave is longer than long wave.2). The distributions of the power spectral densities P_k(ω) in the frequency (ω) domain for k=0-3 and k=5 are computed with the use of (9), as shown in Fig.2. A spectral peak at 40 month period is significant and spectral peaks at 26 and 12 month are clealy seen. For the sake of comparison, the spectral density p_k(ω)for k =5 is also shown in Fig.2. This figure is selected to be representative of long wave. As can be seen in this figure, spectral peak at 40 month is not clealy seen and the spectral density is approximately proportional to l/ω² (dashed).3). In order to explain the characteristic feature of the spectral density of ultr-long wave in the frequency domain,we computed the spectral density of stochastically forced motion in the non-adiadatic condition with the use of the equation(23). The results for k=l-3 are shown in Fig.4. The spectral density is changed with ω and it means that item F_k(t)in the equation(22) is not white noise spectrum. There is a main spectral peak at 40 month and it suggests that the spectral peak of the ultra-long wave at 40 month is driven by the stochastically forced motion.For the sake of comparison, the case of k=5 for the same season is shown in Fig. 4. Although there is a weak fluctuatinn, the spectral density may be regarded as being approximately constant with frequency CD. In this sense the item F_k(t) of stochastical forcing may be regarded as an approximately stationary random process of white noise time series. Therefore the characteristic feature of F_k(t) in the case of long wave is quite different from that of ultra-long wave.