Savers held ¥3 trillion in Japan's temporarily tax-free investment accounts at the end of the program's first year, the Financial Services Agency said in a statement on its website.
That total at the end of December was up from ¥1.6 trillion at the end of June.
The number of Nippon Individual Savings Accounts accounts meanwhile increased to 8.2 million from 7.3 million six months earlier, the agency said.
While the amount of money in the NISA accounts almost doubled in the second half of the year, Daiwa Institute of Research said at the current pace the government won't reach its target of ¥25 trillion in NISA investments by 2020.
That figure would imply an annual pace of about ¥3.6 trillion. The goal was set by the Democratic Party of Japan government in July 2012.
"There have been good increases compared with June," said Shungo Koreeda, a Tokyo-based researcher at Daiwa Institute of Research. "But given their target, they need to speed up."
The data are preliminary, and based on a survey in February of all financial institutions that offer NISA accounts, the FSA said.
Under the program, individuals can invest ¥1 million a year for five years in stocks, exchange-traded funds, investment trusts and real estate investment trusts without taxes on capital gains or dividends. Such levies doubled to 20 percent at the start of last year for investments outside NISA accounts.
A spokesman on NISA at the FSA declined to comment.