Provided is a reactor that includes: a coil; a magnetic core with an inner core portion arranged inside the coil; and an inwardly interposed member insulating the inner core portion from the coil. The inwardly interposed member has a thin portion defined by a recessed inner-circumferential surface, and a thick portion that is thicker than the thin portion. The inner core portion has a core-side projection portion facing the inwardly interposed member that has a shape conforming to the inner-circumferential surface shape of the thin portion. The thin portion has a thickness of 0.2 mm to 1.0 mm inclusive, and the thick portion has a thickness of 1.1 mm to 2.5 mm inclusive. A clearance is provided at least at a portion between the inner core portion and the inwardly interposed member, and the inwardly interposed member and the wound portion are in contact with each other.