The first explosion at Mitsui Chemicals' Iwakuni-Ohtake facility broke windows in more than 100 buildings at about 2 a.m., broadcaster NHK reported. A 22-year-old employee was killed and 16 others injured.
No injuries were reported from a second blast that took place just before 8 a.m., the report said.
Mitsui Chemicals said the explosions happened during a halt in operations that was initiated after problems at a different plant Saturday.
Firefighters were on the scene working to secure the area.