State-owned Japan Post is hoping to raise 1.39 trillion yen ($11.5bn, £7.4bn) in what would be one of the biggest Japanese share sales in the last 30 years.
The Japanese government is hoping that the floatation will boost the country's economy, which contracted in the second quarter, and its stock markets.
"We are hopeful it will lead to a virtuous economic cycle," a senior government official said.
Chief cabinet secretary Yoshihide Suga said the hope is that it would encourage people to take savings out of their bank accounts in put them into shares.
Japan Post employs nearly 200,000 across 24,000 offices in Japan as it controls the country's largest bank and insurer; Japan Post Bank and Japan Post Insurance.
