To prevent sound produced in the respective rooms from being transmitted to the adjacent room through the internal space as a transmission path by insulating sound in the internal space of a rail stretching between adjacent rooms partitioned by a movable partition.

This movable partition device is so constructed that a hollow rail 7 having an opening pointed in the longitudinal direction is disposed on the lower surface of a ceiling of a room, a hanging support rod 14 passed through the opening 12 of a rail 17 is suspended from the lower surface of a runner 15' running the inside of the rail 7, and a movable partition 2' is suspended on the lower end of the hanging support 14, whereby the movable partition 2' is moved along the rail 7 in a room. The movable partition device is so constructed that the upper end part of a sound insulation member 24 substantially forming a complementary shape for the sectional shape of the rail 7 is rotatably pivoted in the rail 7 by a horizontal shaft 22 directly fitted to the longitudinal direction of the rail 7.