This paper describes electricity generation characteristics of the vibration power generator with piezoelectric elements. In general, it is well-known that the above generator has low generation efficiency, having so simple structure. Moreover, vibrations of broad frequency band, which exist in actual environment, bring the characteristics to lower efficiency in comparison with the particular situation in which the host structure is excited by the vibration of the natural frequency. In this study, the vibrations of broad frequency band, i.e. random vibration, attracts attention and mechanical-acoustic coupling with an internal sound field enclosed by the host structures, on which the piezoelectric element is installed, is adopted to improve the electricity generation efficiency. The behavior of mechanical-acoustic coupling is determined by the relationship between the respective characteristics of the host structure vibrations and the internal sound field, so that the electricity generation efficiency depends strongly on those dimensions. Here, the electric power and the electricity generation efficiency are regarded as the representative characteristics of the electricity generation, then the characteristics are considered based on them with changing the dimensions to find the condition in which the efficiency can be improved. As a result, the applicability of such a power generation system in actual environments is mentioned.