A fifth member of the Tokyo 2020 Organising Committee staff has tested positive for COVID-19.
The employee in question is female, in her 40s, and based at an office in Harumi, according to Tokyo 2020.
There have been three other positive cases reported from the same office, with one coming last month and two in August.
The first known COVID-19 case among Tokyo 2020 staff was in April.
"The affected employee has been working from home from 29 September onward; her last day working on site was on 28 September," Tokyo 2020 said in a statement.
"Tokyo 2020 will continue to work with relevant organisations to prevent the spread of COVID-19 and take all necessary measures to maintain a safe and secure working environment for our staff."
After a spike of coronavirus cases in Japan in July and August, the number has been falling again this month.
In total, the country has reported more than 85,700 cases and 1,599 deaths.
The Tokyo 2020 Organising Committee is made up of approximately 3,800 workers from organisations including the Japanese Olympic Committee, the Japanese Paralympic Committee, the Tokyo Metropolitan Government and the Japanese Government.
Some staff began returning to offices in June, with coronavirus countermeasures put in place.
In March, the global health crisis led to the Tokyo 2020 Olympic and Paralympic Games being postponed until 2021.
The Olympics are now scheduled for July 23 to August 8, followed by the Paralympics from August 24 to September 5.
Organisers are currently assessing what actions can be implemented to ensure the Games run smoothly against the backdrop of the pandemic.
A coronavirus countermeasures taskforce, comprised of officials from the Japanese Government, Tokyo Metropolitan Government and Tokyo 2020, is meeting regularly.
A total of 50 measures designed to cut costs, relating to areas including the number of people involved, infrastructure and Ceremonies, have also been approved by the International Olympic Committee (IOC) and Tokyo 2020.
Officials including Tokyo 2020 President Yoshirō Mori, IOC President Thomas Bach and Japanese Prime Minister Yoshihide Suga have appeared confident the Games will take place next year.