A reactor includes a coil formed by connecting a pair of coil elements configured by a wound wire, and magnetic core. The reactor for an on-vehicle converter converts an input voltage. Electricity applying conditions include a maximum DC current of 100 A or higher and 1000 A or lower, an average voltage of 100 V or higher and 1000 V or smaller, and a usable frequency of 5 kHz or higher and 100 kHz or smaller. The magnetic core is obtained by combining a plurality of divided core pieces without a gap provided. All the divided core pieces are made of hardened compact obtained by hardening resin of composite material containing magnetic powder and the resin. A distance between an inner peripheral surface of the coil element and an outer peripheral surface of the magnetic core opposite to the inner peripheral surface is 0.1 mm or more and 2 mm or less.