A substrate with built-in passive element includes passive elements and a substrate. The passive elements include at least one of a capacitor, an inductor, a resistor, a signal transmission element or an optical waveguide element. The capacitor, the inductor, the resistor, the signal transmission element or the optical waveguide element has a functional element filled in a groove-like or hole-like element forming region provided in the substrate along a thickness direction thereof. The functional element has a Si—O bond region obtained by reacting Si particles with an organic Si compound.