An electric linear motion actuator comprising: a housing; a cylindrical linear motion member having a center axis, and slidably incorporated in the housing; a rotary shaft provided on the center axis of the linear motion member, and configured to be rotated by an electric motor; and a rotational motion converting mechanism provided between the rotary shaft and the linear motion member, and configured to convert a rotational motion of the rotary shaft to a linear motion of the linear motion member, wherein the linear motion member comprises: a cylindrical member slidably fitted in the housing, and having an open distal end formed with an opening and located on a side of an open end of the housing; and a pressing member coupled to a distal end portion of the cylindrical member which includes the open distal end of the cylindrical member so as to close the opening of the cylindrical member at the open distal end thereof, wherein the actuator further comprises a boot sealing a space between a distal end portion of the housing and a distal end portion of the linear motion member, wherein a portion of the pressing member coupled to the cylindrical member is located inside of the boot, wherein a boot groove for attaching the boot is formed in an outer periphery of the portion of the pressing member coupled to the cylindrical member, characterized in that the boot groove includes a rear inner side wall defined by the open distal end of the cylindrical member, and has a rectangular section. a housing; a cylindrical linear motion member having a center axis, and slidably incorporated in the housing; a rotary shaft provided on the center axis of the linear motion member, and configured to be rotated by an electric motor; and a rotational motion converting mechanism provided between the rotary shaft and the linear motion member, and configured to convert a rotational motion of the rotary shaft to a linear motion of the linear motion member, wherein the linear motion member comprises: a cylindrical member slidably fitted in the housing, and having an open distal end formed with an opening and located on a side of an open end of the housing; and a pressing member coupled to a distal end portion of the cylindrical member which includes the open distal end of the cylindrical member so as to close the opening of the cylindrical member at the open distal end thereof, wherein the actuator further comprises a boot sealing a space between a distal end portion of the housing and a distal end portion of the linear motion member, wherein a portion of the pressing member coupled to the cylindrical member is located inside of the boot, wherein a boot groove for attaching the boot is formed in an outer periphery of the portion of the pressing member coupled to the cylindrical member, characterized in that the boot groove includes a rear inner side wall defined by the open distal end of the cylindrical member, and has a rectangular section. a cylindrical member slidably fitted in the housing, and having an open distal end formed with an opening and located on a side of an open end of the housing; and a pressing member coupled to a distal end portion of the cylindrical member which includes the open distal end of the cylindrical member so as to close the opening of the cylindrical member at the open distal end thereof, wherein the actuator further comprises a boot sealing a space between a distal end portion of the housing and a distal end portion of the linear motion member, wherein a portion of the pressing member coupled to the cylindrical member is located inside of the boot, wherein a boot groove for attaching the boot is formed in an outer periphery of the portion of the pressing member coupled to the cylindrical member, characterized in that the boot groove includes a rear inner side wall defined by the open distal end of the cylindrical member, and has a rectangular section.