In developing a power-steering system, quietness is one of the important requirements. And a quantitative evaluation of the noise is required to estimate effects of noise reduction practices. Generally, an absolute evaluation based on experienced engineer's audibility is used as an evaluation method of the noise. However, it is difficult to make a detailed evaluation on the noise. Thus, the establishment of the quantitative evaluation technology is required for the detailed evaluation. In this study, we aim to clarify the characteristics of the noise and to establish a quantitative evaluation method for the noise. Sensory evaluation for interior noises of an actual vehicle as well as for artificial noises is conducted by experienced engineers. Then, correlation between the sensory evaluation result and the signal characteristics of interior noises is analyzed, and an evaluation equation for the noise is proposed.