An electric toothbrush comprising: an electric toothbrush main body including a bar-shaped stem which has a leading end part, vibrates by a vibration source provided inside the electric toothbrush main body, and extends along a center axis; and a replacement brush to be attached so as to cover said stem, wherein said stem includes a protruding part region which protrudes outward from an outer peripheral face of the stem, said replacement brush includes: a tubular part having an open trailing end; a brushing member formed outside a leading end side of said tubular part; and an open sidewall formed from a trailing end side toward the leading end side of said tubular part to define an opening region having a leading end side which is closed and a trailing end side which is opened at said open trailing end, said open sidewall has a bulging wall (511 c) protruding toward said opening region, and in the attached state of said replacement brush to said stem, said protruding part region is located inside said opening region, and said bulging wall (511 c) comes into contact with said protruding part region at a trailing end side of said protruding part region, wherein said open sidewall has a first sidewall extending from said open trailing end toward the leading end side, characterized in that the first sidewall is formed so as to extend in an inclined direction relative to said center axis when being seen laterally, said protruding part region has an inclined wall face extending in an inclined direction relative to said center axis when being seen laterally, and in the attached state of said replacement brush to said stem, said first sidewall comes into contact with said inclined wall face. an electric toothbrush main body including a bar-shaped stem which has a leading end part, vibrates by a vibration source provided inside the electric toothbrush main body, and extends along a center axis; and a replacement brush to be attached so as to cover said stem, wherein said stem includes a protruding part region which protrudes outward from an outer peripheral face of the stem, said replacement brush includes: a tubular part having an open trailing end; a brushing member formed outside a leading end side of said tubular part; and an open sidewall formed from a trailing end side toward the leading end side of said tubular part to define an opening region having a leading end side which is closed and a trailing end side which is opened at said open trailing end, said open sidewall has a bulging wall (511 c) protruding toward said opening region, and in the attached state of said replacement brush to said stem, said protruding part region is located inside said opening region, and said bulging wall (511 c) comes into contact with said protruding part region at a trailing end side of said protruding part region, wherein said open sidewall has a first sidewall extending from said open trailing end toward the leading end side, characterized in that the first sidewall is formed so as to extend in an inclined direction relative to said center axis when being seen laterally, said protruding part region has an inclined wall face extending in an inclined direction relative to said center axis when being seen laterally, and in the attached state of said replacement brush to said stem, said first sidewall comes into contact with said inclined wall face. a tubular part having an open trailing end; a brushing member formed outside a leading end side of said tubular part; and an open sidewall formed from a trailing end side toward the leading end side of said tubular part to define an opening region having a leading end side which is closed and a trailing end side which is opened at said open trailing end, said open sidewall has a bulging wall (511 c) protruding toward said opening region, and in the attached state of said replacement brush to said stem, said protruding part region is located inside said opening region, and said bulging wall (511 c) comes into contact with said protruding part region at a trailing end side of said protruding part region, wherein said open sidewall has a first sidewall extending from said open trailing end toward the leading end side, characterized in that the first sidewall is formed so as to extend in an inclined direction relative to said center axis when being seen laterally, said protruding part region has an inclined wall face extending in an inclined direction relative to said center axis when being seen laterally, and in the attached state of said replacement brush to said stem, said first sidewall comes into contact with said inclined wall face.