Environment Minister Yuriko Koike said Friday students applying for jobs had better not wear jackets and ties to the interview.
Students who passed the civil service's written examination must next visit specific ministries and agencies for interviews.
Those applying to enter the Environment Ministry, however, have been showing up in suits despite the government's "Cool Biz" summer dress campaign, which shuns suits and ties.
"It is regrettable that young people go for the 'safe' ways," Koike said at a news conference.
The "Cool Biz" campaign, which started June 3, has been pushed as a way to help reduce greenhouse gas emissions by curbing the use of air conditioning and promoting sales of cooler apparel.