Soundproof cover

A soundproof cover provided in a power seat motor unit that is attached to a frame member of a seat and that has a motor and a gear box that includes a built-in gear assembly, the soundproof cover being mounted to a first gear box, which houses a first gear, of the gear box, the soundproof cover comprising:

• a foam disposed on at least an X-Z surface of the first gear box;
• a cover member disposed on an outer side of the foam;
• and
• a mass body interposed between the foam and the cover member and disposed at a position corresponding to at least the X-Z surface of the first gear box in contact with the foam, in a case where, in the first gear box, an axial direction of the motor is defined as an X direction, a direction of attachment to the frame member, which is one of two directions that are orthogonal to the X direction, is defined as a Y direction, and the remaining direction is defined as a Z direction.