Polling stations opened across most of the Japanese archipelago at 8 a.m. Sunday and will close at 8 p.m.
Although preliminary numbers will be released on late Sunday, the final results are not expected until Monday, reported Efe news.
A total of 370 candidates are contesting these elections where 124 seats will be up for grabs out of the 245 that make up the Upper House.
The remaining seats will be voted for in the next election scheduled for 2022 in which three more seats will be added to raise the total number to 248.
Sunday's elections are seen as a barometer of public support for the government of Prime Minister Shinzo Abe, which has been in power for six-and-a-half years, with Abe on his way to becoming one of Japan's longest-running leaders.
Among the issues that the election campaign has focused on are the rise in the Consumption Tax rate planned for October, the reform of the pacifist article of the constitution pushed by Abe to grant more powers to the Self-Defense Forces, and the sustainability of the national pension system.
The ruling coalition formed by Abe's Liberal Democratic Party and the Buddhist Komeito hope to strengthen their control in the Upper House, where they have held more than 60 per cent of the seats since the last election in 2016.
To promote constitutional reform, which is one of Abe's political priorities, the support of two-thirds of the Upper House is needed, a proportion that is already achieved in the house in its current composition if other parties in favor of the initiative are added.
The Japanese constitution gives more power to the Lower House or the House of Representatives of the parliament (Diet), for which elections are held every four years, and whose decisions prevail over those of the Upper House.
( With inputs from IANS )