With the opposition Democratic Party of Japan predicted to win by a big margin in Sunday's elections -- in which the economy is one of the main issues -- Aso's Liberal Democratic Party heard the jobless rate in July rose to 5.7 percent from 5.4 percent in June, leaving 3.59 million people without employment.
Analysts said the new jobless report is likely to be another setback for the already embattled Aso's government.
The news comes at a time when a recent Kyodo News survey showed the opposition DPJ could win more than 300 seats in the powerful 480-seat lower house of parliament, while Aso's LDP could see its strength drop to just above 100 from its current 300.
The LDP has governed Japan through most of post-World War II years.
Besides the jobless numbers, the world's second largest economy's core consumer price index fell for the fifth consecutive month in July, down by a record 2.2 percent year-on-year, which could raise concerns about deflation.