Parimutuel betting is a betting system employed in gambling on public sports, such as horse racing and greyhound racing. In parimutuel betting, the payoff odds for each outcome are calculated based on the ratios between the amounts wagered on each outcome. Users decide to bet each outcome depending on odds, but they are calculated by an organizer, meaning that no user could detect it if they were manipulated. One possible solution is the use of blockchain to achieve a decentralized betting system. However, previous work does not address the security concerns and operational feasibility of using blockchain for real-world betting systems. In this study, we discuss the advantages and security concerns of blockchain-based decentralized betting systems for horse racing as a case study. We implement such a system as a prototype on a blockchain test network to verify that it actually works. Using our proposed system, we measure gas cost to quantitatively evaluate operational cost. Our experimental result shows that gas cost is kept within a range that was not detrimental to both the organizers and the users.