The brush unit (5) has brush head (7) and coupling sleeve (20). The brush head and the coupling sleeve are interconnected through a tubular neck (6). The hollow space is arranged in inside of coupling sleeve and provided with circular base cross-section. The coupling sleeve is provided with circular segment-shaped wall portion which is thicker than the circular base cross-section and planar axial surface (26) with narrow cross-sectional. The planar axial surface is provided with an axial groove (22). Independent claims are included for the following: (1) handle portion； and (2) electric toothbrush.
