A linear actuator capable of setting a bed to a flat state in emergencies is provided.A linear actuator 10 contains a shaft 16 with a feed male screw section 17, a worm gear reduction device for transmitting the rotation of a motor 40 to the shaft 16, a nut 19 that is coupled by a screw to the feed male screw section 17 and advances or retreats by the forward rotation of the shaft 16 and advances or retreats by the reverse rotation of the shaft 16, and a moving cylinder 12 that is fixed to the nut 19 and advances and retreats relative to a housing 11, wherein a sub-shaft 52 linking to the worm gear reduction device, an inner race 59 spline-coupled to the sub-shaft 52, an engaging male section 82 and engaging female section 83 for free engaging, provided between a collar 55 of the shaft 16 and the inner race 59, an operating ring 66 fitted to allow free rotation on the outer circumference of the housing 11, and a function ring 72 for converting the rotation of the operating ring 66 to an axial movement and transmitting it to the inner race 59 are installed. The shaft 16 can be freely rotated in emergencies by releasing the engaging between the engaging male section and the engaging female section.