Christensen (1997) describes market share turnaround processes, based on intensive case studies of Hard Disk Drive, or HDD industry. Even though established large firms are vulnerable to disruptive innovations, previous models have not explained the underlying mechanism definitively. To address this issue, we develop and use a modified Adner and Zemsky (2005) model. In this non-cooperative game model, two firms are assumed to supply same type of product. One firm, which is assumed to be an initial significant occupant of the market, offers products manufactured with mainly established technologies, and these are superior on the primary dimension. The other firm tries to make horizontal differentiation with new technologies. The demand depends on a net effect which represents switching cost, network externality of existing products, the value created by the new technologies and so on. This enables us to consider various relations between established and new products. So we show an example in which the new product ends up a niche (secondary or isolated) market because of the cost and other advantages gained by the established technology-firm. In contrast, there is a case where the firm adopting the new technology dominates the markets and enjoys the monopoly price. Here, the new technology-product is considered to win high evaluation by consumers. This case also implies that the cost advantage is not the necessary condition for the occurrence of disruptive innovations.