A multi-functional cyclic siloxane compound (A), a siloxane-based (co)polymer prepared from the compound (A), or compound (A) and at least one of a Si monomer having organic bridges (B), an acyclic alkoxy silane monomer (C), and a linear siloxane monomer (D); and a process for preparing a dielectric film using the polymer. The siloxane compound of the present invention is highly reactive, so the polymer prepared from the compound is excellent in mechanical properties, thermal stability and crack resistance, and has a low dielectric constant resulting from compatibility with conventional pore-generating materials. Furthermore, a low content of carbon and high content of SiO.sub.2 enhance its applicability to the process of producing a semiconductor, wherein it finds great use as a dielectric film.