A linear actuator is provided which is able to simplify its structure by adding a stopper function, too, to a detent mechanism and to prevent generation of radial load. The linear actuator comprises a ball screw mechanism, to which a rotational driving force is transmitted, for converting a rotational motion into a linear motion. The ball screw mechanism comprises radially projecting guide projections provided in a linear motion component, and a guide groove arranged in a securing portion facing the linear motion component and is engaged with the guide projections to guide the guide projections in an axial direction. The guide projection has a projection projecting by the predetermined length, while engaging into the guide groove, at a stroke end of the linear motion component and a locking portion provided in a rotary motion component is locked to the projection of the guide projection.