As the mobile ecosystem grows, advertisement (ad) market is shifting to the mobile environment from the PC environment, and the security threats on the mobile environment are increasing. In particular, since mobile ad injection generally exposes other ads into the mobile app rather than proper ad of legitimate advertiser, the advertiser does not achieve expected effect from advertisement, and will lose trust in advertising company. In addition, content providers who post the ads will lose their profit, and users who use the ad-injecting app will suffer from a great inconvenience because of the indiscriminate exposure of ads. Therefore, in this paper, we especially focus on the mobile ad injection that is not much researched, and describe the method to detect ad-injecting app based on review and star rate in Google Play’s app page. GPC (Google Play Crawler) and analyzer are also described in detail. Moreover, effects and limitations of GPC are analyzed based on the crawling result.