The figure is a critical threshold set by Prime Minister Yoshihide Suga to ensure that the nation's sizeable elderly population is covered by the end of July, and all adults by November.
As the campaign has gained momentum, with vaccination sites opening at workplaces on Monday, the latest cabinet office figures show 1,013,061 doses injected on June 14, and just under 1 million on each of the three subsequent days.
Just 18 per cent of a population of 125 million has got at least one dose, a Reuters tracker shows, for the lowest level among major economies, with one month remaining until the start of the Summer Olympics in Tokyo, an event delayed by the pandemic.
The vaccination campaign began in mid-February, trailing most large economies, and initially hampered by scarce supplies of imported doses. But even as shipments increased, shortages of medical staff and logistics hurdles became acute.
To speed things up, the government opened massive vaccination centres operated by the defense ministry and relaxed rules on recipients and those allowed to inject them.