[Object] To provide a polysilazane-containing composition for forming a film. In order to form a dense and processable siliceous film, the composition is intended to be employed in a two-step conversion process comprising the steps of: forming a film having a dry surface from the composition provided that the conversion in to the siliceous substance proceeds insufficiently; and then subjecting the film to secondary processing. [Means] The present invention provides a polysilazane-containing film-forming composition comprising a particular amine compound, a polysilazane compound, and a solvent; and the invention also provides a process for forming a siliceous substance. In the process, the composition is applied to coat a substrate and thereafter converted into the siliceous substance. The particular amine compound has two amine groups, and the amine groups have at least one phenyl-substituted hydrocarbon group.