A reactor is provided that can reduce or alleviate stress that can be applied to a magnetic core, and also has excellent manufacturability, and a reactor manufacturing method is also provided. The reactor includes a coil and a magnetic core that includes a plurality of core pieces and a gap member that is interposed between at least one set of core pieces, the magnetic core forming a closed magnetic circuit when the coil becomes excited. At least one gap member is a resin foam gap member that includes, in a contact region that comes into contact with the core pieces, a resin foam portion constituted by resin foam.