To provide a composition for forming a dielectric layer excellent in dielectric constant and withstand voltage properties, a MIM capacitor and a process for its production.A composition for forming a dielectric layer, which comprises fine particles of perovskite type dielectric crystal, glass frit, and a hydrolysable silicon compound or its oligomer, and a MIM capacitor comprising a substrate, and a bottom electrode layer, a dielectric layer having a structure such that fine particles of perovskite type dielectric crystal are dispersed in a silicon oxide matrix containing glass-forming ions and a top electrode, formed on the substrate in this order.