Since Broadbent presented a model of attention mechanism, the so called filter theory (1958), many studies on this problem have been done and two international symposia have been devoted explicitly to this problem. This paper attempts to review theories of attention briefly and to discuss a few of our recent studies on attention distracted by an unconscious process. The study of the phenomenon of attention is the study of the limitation and selection in the central nervous system which processes incoming informations. Respiratory activity is spontaneous physiologically and in normal situations it is an unconsious process in psychological significance. We found in simple reaction time measurements that the RT increased some 20 msec, when a stimulus for RT was given at the beginning of inspiration in a respiratory cycle and that the reguration occured in the respiratory rhythm when the signal for RT was expected. These results indicate that the processing of the RT may be blocked while the central nervous system which is supposed to be that of single channel is busy to carry out breathing even though it proceeds without being aware of it. This reguration of the rhythm suggests a mode of selection process in the central nervous system.