Downsized and weight reduced reactor is provided. An annular core 3 includes a first leg 31 and a second leg 32 to which a coil 2 is mounted and which generates magnetic flux, and a pair of yokes 33 which form a closed magnetic path together with the legs. Recess portions 35a is formed at four corners of the annular core 3, and a part of the end surfaces 31a and 32a of the legs which is a magnetic flux generating part or an end surface of the magnetic flux generating part is exposed from the recess portion 35a.