The U.S.-based company operates 25 vehicles in nine cities including Arlington, Texas, and Hiroshima, Japan.
Over the next two years, May will use the money to advance its self-driving software to the point that it can remove human safety drivers from shuttles and replace them with remote supervisors who can monitor several vehicles at once.
Increasing the ratio of robotic cars to humans will allow May to become profitable, executives said.
The Series C funding round was led by Mirai Creation Fund II, managed by Sparx Group. New investors included insurance company Tokio Marine Holdings, and Toyota Tsusho, Toyota's trading arm. The company has raised $166 million in total and says it has provided more than 300,000 revenue-generating rides.
May sells its low-speed shuttle services to cities and businesses, rather than pursuing a robotaxi model pursued by more deep-pocketed peers like Waymo and Cruise.
This year it plans to convert its fleet from hybrid Lexus SUVs to hybrid Toyota Sienna minivans.