Overview of Instant Replay in NPB
NPB introduced video review on a trial basis for home run calls in 2010, then launched a formal replay review system covering home run and foul-line decisions in 2014. This followed MLB's trajectory of adopting home-run-only replay in 2008 and expanding to a full challenge system in 2014. Under NPB's system, the umpire crew would walk to a monitor room behind the dugout to review footage. In the inaugural 2014 season, 47 reviews were conducted and 19 calls were overturned, a reversal rate of approximately 40 percent that demonstrated the system's effectiveness. To minimize game-time impact, a five-minute cap was placed on each review.
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Technological Evolution of the Review System
Initially, each stadium was equipped with roughly six standard-definition cameras. Upgrades to full HD began in 2016, and by 2018 high-speed super-slow-motion cameras had been installed at major venues. Tokyo Dome and Koshien Stadium each deployed more than 12 cameras, virtually eliminating blind spots. In 2020, the COVID-19 pandemic accelerated a remote-review pilot program, establishing a centralized video center at NPB headquarters in Tokyo capable of monitoring all stadiums in real time. Remote review became the official protocol from the 2022 season, eliminating the need for umpires to leave the field. Average review time dropped from 3 minutes 12 seconds to 1 minute 48 seconds.
Accuracy and Operational Challenges
In the 2023 season, 82 replay reviews were conducted with a 35 percent overturn rate. Review scope expanded in 2018 to include collision-rule verification alongside home run and foul calls. However, ball-strike calls and check-swing appeals remain outside the system. About 25 percent of reviews result in a confirmed call due to inconclusive camera angles. Some fans advocate for an MLB-style manager challenge system, but NPB has maintained a cautious stance prioritizing game tempo. A notable controversy arose during Game 5 of the 2019 Japan Series between SoftBank and the Giants, when a play-at-the-plate review lasted 4 minutes 30 seconds and was criticized for disrupting the game's flow.
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Future Outlook and Automated Umpiring
NPB began testing the Automated Ball-Strike System (ABS) in its minor leagues in 2024, employing ball-tracking technology from TrackMan and Hawk-Eye similar to the system MLB deployed across its minor leagues in 2023. A first-team rollout is tentatively targeted for 2026 or later. Research into AI-assisted tag-play adjudication is also underway, combining high-resolution cameras with sensor technology to resolve calls at the millimeter level beyond human perception. The KBO's full adoption of ABS at the top level in 2024 is expected to influence NPB's decision-making timeline. The continued evolution of video review technology serves as an indispensable complement to umpires, advancing the goal of fairer game administration.