Masking of low frequency sound by band noise was measured and the masking characteristic by band noise was investigated. The center frequency of band noise is 20 Hz. Forty subjects participated in the experiments. As a result of the experiments, the frequency characteristics of masking by band noise were divided into three groups by a cluster analysis in multivariate analysis. The first group is that the masking values are almost positive at all frequencies. The second is that they are very small or negative at several frequencies. The third is that they are positive at frequencies lower than 20 Hz and negative at frequencies higher than 20 Hz like inverse S shape curve. To find the reason why the subjects are divided into three groups on the masking characteristics, psychological experiments were carried out. Many kinds of sound stimuli which are pure tones, band noises and both pure tones and band noises were prepared. The results show that psychological responses were different in each group to some sound stimuli. The relation between masking characteristics and psychological responses were detected. Masking mechanism at low frequencies may not be the same as that at audio frequencies.