The mutational falsetto is failure to change from the higher pitched voice of preadolescence to the lower pitched voice of adolescence and adulthood. The one of characteristic findings is contraction of suprahyoid muscles. The large majority of young men with inappropriately high voice have excellent voice therapy prognosis. We have experienced one case of mutational falsetto treated with botulinum toxin injection on suprahyoid muscles. His suprahyoid muscles are contracted markedly simultaneously with each phonation. fundamental frequency$(F_0)$ of his vowel phonation was 332Hz. Extensive voice therapy including manual compression of thyroid notch was ineffective. Forty units of Botox$^{circledR}$ was injected under the EMG-guidance(20U bilaterally). At seven days post-injection, his voice changed lower than before and at 40 days after procedure, his $F_0$ was 126Hz.