Representatives from the 12 teams made the decision during an online meeting on Monday afternoon.
The NPB season was supposed to start on March 20 but has been delayed because of the coronavirus.
With Japan's nationwide state of emergency having been lifted region by region, based on infection rates, J-League clubs now have an uneven level of preparedness.
"I hope we can provide some guidance for sports other than professional baseball", Saito said.
These are good signs for the authorities with respect to the Olympics as International Olympic Committee (IOC) president Thomas Bach had earlier conceded that the Games will have to be cancelled if it can not take place next year. NPB teams will aim to play 120 games in their condensed season, which represents a reduction from the 143-game norm.
Japanese Prime Minister Shinzō Abe today lifted the state of emergency in the five prefectures where it was still in place - Tokyo and neighbouring Chiba, Kanagawa and Saitama, as well as the northern island of Hokkaido - allowing for baseball games to now be played behind closed doors. And while NPB will formally begin a new training camp that runs June 2-14, Nagatsuka adds that teams are already beginning intrasquad games at their home parks in an effort to build back up.
"It is with great joy that we have been able to decide on opening the season".
Nippon Professional Baseball Commissioner Saito Atsushi said he is sorry that the sport's governing body had to change the schedule several times and kept players and fans waiting.
The Korea Baseball Organisation League has also begun behind closed doors.
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