Tatsutoshi Goto
Immediately after leaving AJPW, Nagai was signed by New Japan Pro-Wrestling (NJPW), whom he had previously worked for during the inter-promotional relationship between both promotions. Due to his background in kickboxing and Rings, Nagai was placed into the Makai Club, a group of wrestlers with legitimate backgrounds in martial arts. Nagai debuted under a mask as Makai #5 at Wrestling World 2003, teaming with the returning Katsuyori Shibata, who was now masked and competing as Makai #4. In their debut as a team, the two defeated Nagai's former partner Masahito Kakihara and Takashi Iizuka. After defeating Kakihara in singles action at Ryogoku Kokugikan in February, Makai #5 voluntarily unmasked himself as Nagai, though he continued to compete as both himself and Makai #5 after this. In July, Makai #5 and #4, who had dubbed themselves , challenged for the IWGP Tag Team Championship, losing to reigning champions Hiroshi Tanahashi and Yutaka Yoshie in Osaka. Nagai competed at Wrestling World 2004, teaming with Makai #1, Ryushi Yanagisawa and Ryota Chikuzen to defeat the Crazy Dogs (Enson Inoue, Hiro Saito, Michiyoshi Ohara and Tatsutoshi Goto). Nagai would compete twice at NJPW's King of Sports pay-per-view in March, first teaming with Shibata, #1 and Yanagisawa to defeat Blue Wolf, Shinya Makabe, Toru Yano and Yutaka Yoshie, but later losing to Josh Barnett in a singles match.