Audio CAPTCHA systems, which distinguish between software agents and human beings, are especially important for persons with visual disability. The popular approach is based on mixing-based methods (MBM), which use the mixed sounds of target speech and noises. We have proposed a deletion-based method (DBM) which uses the phonemic restoration effects. Our approach can control the difficulty of tasks simply by the masking ratio. In this paper, we propose a design principle of CAPTCHA, according to which the tasks should be designed so that the large difference of performance between the machines and human beings can be provided. We also show the experimental results that support the hypotheses as follows: (1) only using MBM, the degree of task difficulty can not be controlled easily, (2) using DBM, the degree of task difficulty and safeness of CAPTCHA system can be controlled easily.