To provide a hollow actuator capable of preventing hooking or damage of pipes, hoses and lead wires inserted through or housed in the inner diameter of a hollow shaft, while reducing or preventing overheating in the actuator.

The hollow shaft structure 10 of a hollow actuator has a structure including a hollow actuator 8. The internal surface 41 of the sensor built-in portion 4 is formed flush with the inner diameter surface 21 of a hollow shaft 2, and a void V is provided between the end 23 of the hollow shaft 2 and the hollow shaft side end 43. The hollow actuator 8 may includes a decelerator.