Mental effort is a limited resource which must be invested to perform mental tasks. The amount of mental effort investment that an individual experiences during task performance can be measured afterwards with the help of self-rating scales. Earlier research suggests that integration of information about somatic state changes is crucial for the self-evaluation of mental effort investment. Damage to the pathways which convey information about somatic state changes can lead to an inability to self-evaluate mental effort investment, while conceptually similar evaluations of task difficulty can still be performed. We used functional magnetic resonance imaging to investigate brain activation, while subjects rated their mental effort investment and the difficulty of a previously performed task. Our results show stronger activation of the left anterior insular cortex (aIC) during evaluation of mental effort. Additionally, the activity in left aIC during task performance was modulated by changes in task demand in a similar way as the self-ratings of mental effort. We argue that aIC does not only play a role in the integration of self-related information during self-evaluation of mental effort investment, but that left aIC might also be involved in the experience of mental effort during task performance.