A multilayer flexible substrate includes a first structural layer including at least one resin sheet including an insulating layer, a wiring conductor provided on a principal surface of the insulating layer, and filled vias disposed in the insulating layer; and a second structural layer provided on a principal surface of a portion of the first structural layer and including at least one resin sheet including an insulating layer, a wiring conductor provided on a principal surface of the insulating layer, and a filled via provided in the insulating layer. The multilayer flexible substrate includes rigid regions and a flexible region that is more flexible than the rigid regions. In the multilayer flexible substrate, the filled via disposed in the flexible region has a higher porosity than the filled via disposed in the second structural layer.