Individuals with adult attention deficit/hyperactivity disorder (ADHD) have a deficit in their cognitive control. The aim of this study was to reveal the brain correlates of the deficits in response inhibition or error processing in adult ADHD. A total of 29 adults with ADHD and 25 control individuals were recruited. They completed an event-related-design Go/No-go task under functional magnetic resonance imaging scanning. Both the ADHD group and the control group exhibited activation of the frontostriatal network when processing response inhibition. They also exhibited activation of the frontoinsula cortex and anterior cingulate in error processing. Adults with ADHD have a lower brain activation of error processing over the right inferior frontal lobe adjacent to the insula than control individuals. The altered frontoinsula cortex activation may represent the mechanism of error processing deficit among adults with ADHD.