One of the key technologies for natural man-machine interface is hands-free speech recognition. The performance of hands-free distant- talking speech recognition will be seriously degraded by noise and reverberation in real environments. A microphone array is applied to solve the problem. When applying a microphone array to speech recognition, parameters such as number of microphone elements and their spacing interval affect the performance. In order to optimize these parameters, a measure which reflects recognition performance is needed. In this paper, we investigate a measure of a microphone array design for speech recognition through experiments using various kinds of a microphone array design.