In ultra-low field MRI with optically pumped atomic magnetometer (OPAM), it is required for the direct detection of MR signals with high sensitivity to match the resonant frequency of potassium to that of proton. A ferrite shield box, which has high-permeability and high electric resistance, is able to be utilized to match the resonant frequencies. In this study, analyses of magnetic field distributions in the vicinities of sample and potassium vapor cell of OPAM were carried out. In addition, the uniformities of the magnetic fields in the areas of sample and potassium vapor cell of OPAM and the magnetic field caused by MR signals were evaluated. These analyses indicated that the resonant frequencies of proton and potassium were matched by using the ferrite shield box with a number of combinations of height and thickness. The uniformities of the magnetic fields applied to a sample were found to be improved by placing the ferrite shield box far from the sample. Furthermore, regardless of the height, the magnetic field caused by MR signals was able to be measured.