Objectives The purpose of this study was to investigate the difference of brain activity between internet game overusers in adulthood and normal adults in a state of disembodiment. Methods The fMRI images were taken while the internet game overuser group (n = 14) and the control group (n = 15) were asked to perform the task composed of ball-throwing animations. The task reflected on either self-agency about ball-throwing or location of a ball. And each block was shown with either different (changing viewpoint) or same animations (fixed viewpoint). The disembodiment-related condition was the interaction between agency task and changing viewpoint. Results 1) In within-group analyses, the control group exhibited higher brain activation in the left precentral gyrus, the left inferior frontal gyrus, and the left insula. And the overuser group exhibited higher activation in the right cuneus, the left posterior middle occipital gyrus, and the left parahippocampal gyrus. 2) In between-group analyses, the control group exhibited higher activation in the right posterior superior temporal gyrus. And the overuser group exhibited higher activation in the left cuneus, and the left posterior middle occipital area. Conclusions These results show that the disembodiment-related brain activation of internet game overusers in adulthood is different from that of normal adults.