A linear actuator which can make a bed flat in emergency. A linear actuator (10) comprising a shaft (16) having an externally threaded portion (17), a worm reduction gear for transmitting rotation of a motor (40) to the shaft (16), a nut (19) screwing on the externally threaded portion (17) and advancing or retreating as the shaft (16) rotates forward or reversely, and a moving tube (12) secured to the nut (19) and advancing or retreating for the housing (11) is further provided with a sub-shaft (52) interlocked with the worm reduction gear, an inner race (59) spline coupled with the sub-shaft (52), an engaging male portion (82) and an engaging female portion (83) interposed between the inner race (59) and the collar (55) of the shaft (16) to engage freely with each other, an operating ring (66) fitted rotatably to the outer circumference of the housing (11), and a working ring (72) for transmitting the rotation of the operating ring (66) to the inner race (59) while converting into axial movement. The shaft (16) can be rotated freely in emergency by disengaging the engaging male portion and the engaging female portion.