A seal belt (4) includes an inner belt segment (41) and an outer belt segment (42); the inner belt segment (41) is configured in a manner so as to revolve alongside the motion of a moving body (2); the outer belt segment (42) contains a first section (43) of which one end (43a) is held at the side section of the top end (10a) of the internal section (21a) of the moving body (2) between the inner belt segment (41) and an aperture (10), the remaining portion being held at a third space position (P3) and extending along the aperture (10), and a second section (44) of which one end (44a) is held at the side section of the bottom end (10b) of the internal section (21a) between the inner belt segment (41) and the aperture (10), the remaining portion being held at a fourth space position (P4) and extending to the fourth space position (P4) along the aperture (10), and the outer belt segment (42) is configured in a manner such that the length of the first section (43) and the length of the second section (44) change in a complementary manner alongside the motion of the moving body (2); and a drive mechanism (3) is positioned in a space encircled by the inner belt segment.
