Provided are: an isocyanate composition for producing a flexible foam that has good vibration absorption as a result of having a low rebound resilience rate and a low hysteresis loss rate even in a low-density area; a composition for forming a flexible foam in which the isocyanate composition is used; and a flexible foam production method in which the composition for forming a flexible foam is used. The composition for forming a flexible foam has a low hysteresis loss rate and high durability in, a high hardness range and in a low density range. The flexible foam production method uses this composition for forming a flexible foam. The flexible foam production method does not cause odor problems or the like, maintains the durability required of a flexible foam even when the density thereof is reduced, and achieves good riding comfort properties and high safety. The problem addressed by the present invention is solved by the use of at least a specific modified polyisocyanate composition, the use of a specific polyoxyalkylene polyol, and the use of a reactive amine catalyst.