Provided is a reactor having a small size with consideration of loss reduction. A reactor 1A includes a coil 10 and a magnetic core 20. The coil 10 is formed by winding a wire. The magnetic core 20 includes an internal core portion 21 that is inserted through the coil 10 and a couple core portion 23 that is coupled to an end of the internal core portion 21 and that covers an outer periphery of the coil 10. The core portions 21 and 23 form a closed magnetic path. An interposed core portion 25 is disposed between the coil 10 and the internal core portion 21. The reactor 1A satisfies 0 B2 and B1>B3, where B1 is a saturation magnetic flux density of the internal core portion 21, B2 is a saturation magnetic flux density of the couple core portion 23, and B3 is a saturation magnetic flux density of the interposed core portion 25.