PURPOSE: To provide an easy and economical sound insulating and cooling apparatus for a rotary driving source by controlling the opening and closing of a casing door part by a controller and ventilating a casing.

CONSTITUTION: An operational panel 24 is operated, a cylinder 32 is energized by an electric signal of a controller 22, a door part 34 of a casing 14 is opened while rotated by a prescribed revolution angle around the axis of a bearing member 30, and a first open part 74 and a second open part 76 are formed dividually in the casing 14 by partitioning. Then, electricity is applied to a motor part 50 of a d.c. electric dynamometer 16 to drive a rotor 53 and a fan 52. External air is introduced into a first chamber 78 through the open part 74 by the driving of the fan 52, the air passes a slit of a cover member 60 and goes toward the rotor 53 side, the rotor 53 is cooled. and deterioration of magnetic force properties is thus prevented. Heat generated in the rotor 53 is discharged to the outside through the open part 76. In this way, the motor part 50 can be cooled by external cool air passing through the open part 74 and the heat of the motor part 50 can be led to the outside of the casing 14 through the open part 76.