A regular in the Japanese championship, Nagashima was second in the J-GP3 category in 2012 and sixth in J-GP2 at the helm of a TSR in 2013. It was with this bike with which he made his World Championship debut at the Japanese Grand Prix, with the JiR Team; he finished the race in 20th position. This experience would prove to be critical, as Nagashima was selected to race for the team full-time when compatriot Kohta Nozane elected not to compete across the 2014 season. After coming second in Moto2™ in the FIM CEV Repsol in 2016 and taking a first win in the series, Nagashima moved up to the Moto2™ World Championship in 2017 with Teluru SAG Team. With some solid finishes under his belt, he switched to Idemitsu Honda Team Asia in the class for 2018 and claimed eight point scoring finishes. The Japanese rider moved to the SAG Team for 2019 and made a further step by regularly competing inside the top ten and even claiming two top five finishes. A debut pole position at the Red Bull Ring and two further front row starts along the way earned a ride with Red Bull KTM Ajo for 2020 with the mission of fighting for podiums. He started the year in stellar form with a win in Qatar and a podium at Jerez, but faded and found himself without a seat for 2021, a year in which he would take part in three Grands Prix - Austin, Algarve and Valencia - as a replacement. Joining the Honda test team, he took part in the Suzuka 8 Hours in 2022, playing a key role in the golden wing brand's first win since 2014. The prize would double as he got get his chance as a wildcard in MotoGP™ at his home GP at Motegi.