In 2015, he directed the feature film The Anthem of the Heart which received a nomination for Animation of the Year at the 39th Japanese Academy Awards.

After graduating from Niigata College of Art & Design, Nagai worked as a freelance storyboard artist and director in the anime industry in 2000.

[1][2] During this time he met screenwriter Mari Okada, and character designer Masayoshi Tanaka, with whom he formed an artist collective called Super Peace Busters.

[3] In 2008, Nagai worked as part of the Super Peace Busters and directed Toradora!, an anime adaptation of Yuyuko Takemiya's light novel series of the same name.

[7] In 2015, Super Peace Busters reunited to work on the feature film The Anthem of the Heart (directed by Nagai).