We studied the mechanism of the magnetic anisotropy in distorted bct-Fe₅₀Co₅₀ ordered alloys layered on Rh using a first principles calculation. For this purpose, we evaluated the uniaxial anisotropy constant K_u and decomposed it into the bulk contribution K_v and the interface contribution K_int. We discussed the origins of these contributions from the site-resolved K_u. The K_v of the [Fe/Co]/Rh almost reproduces the K_u of the bulk bct-Fe₅₀Co₅₀ in the case of both Fe/Rh and Co/Rh interfaces because the K_u value at the inner layers of the [Fe/Co] is almost the same as that at the corresponding atoms in the bulk FeCo. As for K_int, it has a negative value in the two interface cases because the K_u value of the Fe and Co atoms at the Rh interface is almost reduced to zero, which is primarily due to the collapse of the characteristic band structure in the [Fe/Co] (c/a ＞ 1) by the mixing of electronic states with the Rh atoms.