Mega platform operators are active in the advertising market. They make use of that profit to provide new services to consumers. In that respect, online advertising is at the core of the Internet ecosystem. Today many competition authorities are carrying out sector inquiries on this topic. There is one common concern they share is lack of transparency in the online advertising market, especially in the programmatic display advertising market. This paper classifies ad tech markets in order to analyze the advertising industry from the perspective of competition policy and sorts essential issues from the practices and discussions in foreign countries. Significant results of this study are as follows. First, competitive advantage in the advertising market is determined by ad tech, data, and advertisement inventory. Google seems to control all of these. Second, the publisher-side ad servers are considered a bottleneck for competition among ad exchanges. Third, sequential auctions carried out by Google create arbitrage opportunities for Google. These insights have implications for public policy and enforcement of competition law in Japan.