To provide a hollow actuator having excellent environmental resistance, in which it is possible to make use of its advantages by limiting the length of the actuator so that the actuator has a form as flat as possible, and to provide its sensor arrangement structure.

A sensor arrangement structure 10 is an arrangement structure of sensors provided in a hollow actuator 8. A magnetic sensor 1 and a resolver 3 are arranged in a direction orthogonal to the axis 2 of the actuator 8. The resolver 3 is outer rotor type, and fixed so that the rotor 34 of the resolver 3 is located on the inside of the gear 12 of the magnetic sensor 1.