We experienced three cases of high signal intensity observed by Brain Magnetic Resonance Imaging in CO2 arc welders of steel-frame manufacturing industry. Case 1 was a 35 years old man who has been an CO2 arc welder for 10 years, admitted a sudden onset of tonic clonic seizure. He complained fever, chilling and myalgia since 3 days before admission. On admission, in the test of manganese exposure indices, manganese concentrations of blood, urine and scalp hair were 5.17 microgram/dL, 22.00 microgram/l and 31.25 ppm respectively. Case 2 was a 35 years old man who has been an CO2 arc welder for 20 years. On admission, He complained fatigue, numbness and weakness of extremities, and decrease of libido. In the test of manganese exposure indices, manganese concentrations of blood, urine and scalp hair were 6.34 microgram/dL, 14.62 microgram/l and 57.87 ppm respectively. In neurologic examination, Palmo-mentis reflex and Myerson sign were observed. Case 3 was a 33 years old man who has been an CO2 arc welder for 16 years. On admission, He complainer loss of appetite, numbness of extremities, fatigue and decrease of attention. In the test of manganese exposure indices, manganese concentrations of blood, urine and scalp hair were 5.14 microgram/dL, 13.79 microgram/l and 50.08 ppm respectively. In neurologic examination, Myerson sign was observed. In brain magnetic resonance imaging, T1WI showed symmetrical high signal intensity in basal ganglia and midbrain of three cases. Authors argued that they were developed by manganese exposure, and we considered that follow up study would be necessary.