To provide an infill forming system which can provide an indoor space excellent in sound insulation, when an office building is converted into a dwelling, for instance.

The infill forming system is comprised of an infill unit (1) for defining the indoor space, a suspension member (2a) for suspending the infill unit from a structural skeleton (10), and a swing preventing member (3) for preventing swing of the infill unit. The swing preventing member has a damper (3a) arranged between a wall portion (12) of the structural skeleton and the infill unit, and a damper (3b) arranged between a floor portion (13) of the structural skeleton and the infill unit.