In this paper, the minimum audible sound pressure of pure tone of 4,000Hz masked by one octabve band noise of center frequency 4,000 Hz is measured by means of the method of constant, and the measurement of the method of constant is compared with that of the method fo limit. As a result, it is clarified that the masking noise more than 40 dB effects the minimum audible sound pressure. In case of the method of constant, even if the masking noise level increases, the frequency distribution of the degree of judgement to pure tone dose not have bimodality, and catastrophe phenomenon does not occur.