A reactor includes a coil, a magnetic core having an inner core portion inside a winding portion, and an inner interposed member insulating the winding portion from the inner core portion. The inner interposed member includes a thin portion with a small thickness formed by a recess, and a thick portion with a thickness larger than that of the thin portion. The inner core portion includes a core-side projecting portion with a shape conforming to a shape of the inner peripheral face of the thin portion. The thickness of the thin portion is 0.2 mm or more and 1.0 mm or less, and the thickness of the thick portion is 1.1 mm or more and 2.5 mm or less. Clearances are in part of a portion between the inner core portion and the inner interposed member and of a portion between the inner interposed member and the winding portion.