In wireless communication systems, the importance of standardization has been recognized for a long time. In this paper, an analysis is made of the factors which affect promotion and support of the standardization scheme for wireless communication systems by Japanese firms. The standardization process is split into four phases, pre-standardization, early standardization, mid-standardization, and post-standardization periods, and major factors are assumed and validated in each phase. The five factors are a) technical superiority, b) awareness of technology, c) role of leading firm, d) external cooperation and e) firm's competences. Having conducted in-depth interviews at ARIB (Association of Radio Industries and Businesses), a questionnaire to ARIB members was carried out and 12 responses were received from the key firms. From the statistical analyses, different factors, depending on the phase of standardization process, were identified. The importance is put on technological superiority in the pre- and early standardization phases, while the firm's competence and customer requirements were identified to be important in the mid-standardization phase. Some differences were identified between the operators and communications equipment makers.