A second victory for the Brave Blossoms, who lit up the tournament with a stunning upset over South Africa on the opening weekend, kept Eddie Jones' side in contention to reach the quarter-finals for the first time.
They sit third in Pool B and will need to beat the United States in their final match in Gloucester on Sunday and hope other results go their way.
Rugby fever has gripped Japan, who will host the 2019 tournament, and the 26-5 victory over Samoa in Milton Keynes underlined their emergence as a force in the game.
Saturday's broadcast figure eclipsed the previous record of 20.7 million held by France, when national broadcaster TF1 televised the first semi-final of 2007 Rugby World Cup between France and England in Paris.
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World Rugby chief executive Brett Gosper said: "This is a significant result for the game in Japan. It shows that the general population there appreciate the sport and the amazing performances of their national team over the past few weeks.
"Their style of play has really caught the imagination of rugby fans around the world and, as these broadcast numbers indicate, in Japan."
(Reporting by Justin Palmer; editing by Nick Mulvenney)
