PURPOSE: To enable operation to be effected even in corrosive medium under hot circumstance by arranging a cylinder consisting of a magnetic material at the inside periphery of a stator iron core, and surrounding a stator, inclusive of a coil, with a frame and a cylinder thereby putting it in completely sealed condition.

CONSTITUTION: In side a frame 9 is arranged a stator 1, where a coil is wound in the slot 4 of the stator iron core 2 consisting of a magnetic material. Moreover, a cylinder 7 consisting of a magnetic material, which has magnetic property low in saturation magnetic flux density, is arranged at the inside periphery of the stator iron core 2. The stator 1 including the coil 3 is completely sealed by surrounding it with a frame 9 and the cylinder 7, thus the stator 1 is isolated from outside environment. The cylinder 7 is made of a magnetic material 0.5-0.8(T) in magnetic flux density, 100 or over in relative permeability, and besides 1.6(T) or over in magnetic flux density, and 100 or over in relative permeability. Hereby, this can operate even in corrosive medium under hot circumstance at, for example 200°C.