Auditory scene analysis is essential in daily life to extract necessary information from complex acoustic environment and also from intricate development of music compositions. Auditory illusions and ambiguity are important factors in auditory scene analysis and have been studied extensively. We here report a novel form of ambiguity involving two illusory melodies implied by a very simple stimulus consisting of two sustained tones of different frequencies and an intermittently repeated tone of a frequency between the sustained tones. The measured time elapsed before a first perception change showed that illusion, ambiguity and disambiguation actually took place. We anticipate that the proposed illusion and ambiguity will provide a well-controlled approach for behavioral and neurophysiological studies of music recognition because of the simplicity of stimulus.