The bush warblers (Cettia diphone) have been recognized to possess two types of songs: a normal song that plays roles in attracting mate and territorial defense, and an anomalous song. The present study suggests that the anomalous song functions as an alarm signal as well as other unknown signals. Field observations and playback experiments on the anomalous song of bush warbler were conducted in order to investigate the contextual information that occurred between sender and receiver. In the field observation, the males frequently emitted anomalous songs to potential predators. The males responded with an anomalous song to stuffed potential predators. The distance from where the anomalous song occurs to the stimulating source varied depending upon the kinds of stimulus. The males of bush warbler possibly show different responses to the anomalous song depending on the level of danger. When the anomalous song was played back to terrestrial males and females, no distinctive behavior was observed. The anomalous song may be sung to defend the territory against predators or to distract invaders from the nest and female because the male and female behaviors were related with the anomalous song and its phonetic characteristics.