A connection structure for a wiring substrate and a flexible substrate including a wiring substrate and a flexible substrate, in which the wiring substrate includes an insulating member, conductor layer, and ground layer, the flexible substrate includes an insulating sheet and metal film, and the metal film includes a signal line pad joined to the conductor layer via a joining material when viewed from the back surface of the flexible substrate. When viewed from behind the flexible substrate, there is an overlap region where the signal line pad and conductor layer overlap. In a cross-section when the overlap region is cut in a direction perpendicular to a signal transmission direction, in a case where a width of the signal line pad including the overlap region is W, and a width of the conductor layer including the overlap region is W0, the connection structure satisfies W0