To provide an retrofitting type ferrite core which can be fixed at a low cost having proper operativity and shows stable proper characteristics holding of a fixing position.

An retrofitting type ferrite core has a pair of core cases 1a, 1b for storing a pair of ferrite cores 2a, 2b, which are shaped to expose a cut surface 17 of a halved tube with a through-hole 18, with the cut surface 17 made to face it. One end of a pair of core cases 1a, 1b is connected mutually by means of a hinge 5, and the other end is provided with a clamper 3 and a hook 4 for mutual fixation. A conducting wire is disposed at a position of the through-hole 18 and a pair of core cases 1a, 1b are fit and the other end is secured. Thereby, recessed parts 7a, 7b which serve as grooves 7 are formed outside a pair of core cases 1a, 1b respectively in the retrofitting type ferrite core wherein a conducting wire is disposed in the through-hole 18.