North Korea fired a ballistic missile toward the sea east to the Korean Peninsula on Saturday, the South Korean military said.
It is the latest in an unprecedented number of missile tests conducted by North Korea this year.
Japan's coast guard said North Korea fired what could be a ballistic missile and sent notice about a second possible missile.
North Korea fired the missile the day after South Korea's defense ministry announced it had successfully conducted a test flight of a solid-propellant space launch vehicle.
On Monday, five North Korean drones had crossed into South Korea, prompting South Korea's military to scramble fighter jets and attack helicopters to try to shoot them down, in the first such intrusion since 2017.
Not counting Saturday, North Korea fired around 70 ballistic missiles about 38 times this year, Yonhap news agency said, including about eight intercontinental ballistic missiles (ICBM).