The electronic structures of metalloid elements, X (X = B, C, N) existing at the octahedral interstitial site in bcc Cr were calculated by the molecular orbital method. The covalent interaction between X and Cr atoms changed in the order B > C > N, whereas the ionic interaction changed in the reverse order. It was shown that the first-nearest-neighbour Cr atoms moved away from the metalloid element at the centre, by about 40% in the unit of half the lattice parameter. As a result, a nearly regular octahedron of Cr atoms was formed locally around the metalloid element. Such a calculated X-Cr distance in bcc Cr was found to be observed commonly in various crystal structures of compounds.