An extensible flexible printed circuit board sets one or a plurality of flexible printed circuit boards each including a plurality of conductive layers and an insulating layer as a base circuit board, and includes: component mounting parts which are provided at least at parts of the base circuit board, and capable of mounting electronic components; an extensible conductive part which is provided at least at a part of the base circuit board, includes a plurality of joint parts each intersecting a center line in an extension/contraction direction and a plurality of curved parts each continuing from an end part of each joint part and curve, and exhibits extensibility by the curved parts curving to open or close; and a covering member whose material is a flexibly deformable elastomer, and which covers the component mounting parts and the extensible conductive part.