A coil built-in multilayer substrate includes a multilayer substrate, a coil, interlayer connection conductors, and gaps. The multilayer substrate includes a magnetic layer, a component-mounting land conductor provided on a first principal surface, and a terminal conductor provided on a second principal surface. The coil is provided in the magnetic layer and includes an axis extending in a direction perpendicular or substantially perpendicular to the first and second principal surfaces. The interlayer connection conductors are provided in the magnetic layer in a region inside the spiral coil. Gaps penetrate lateral surfaces of the interlayer connection conductors.