The microstructures of Ca-La-Co M-type sintered ferrite magnets were analyzed by Spherical Aberration Corrected Scanning Transmission Electron Microscopy (Cs-STEM) to devise guidelines for improving performance. It was confirmed that there were Ca-Si-based oxides, which consists of Si, Ca, La and Fe, at multiple-junction phases of the sintered body, after adding only SiO2 instead of both CaCO3 and SiO2, which are sintering aids for ferrite magnets. It was also confirmed that the ratio of Si, Ca, La and Fe at multiple-junction phases was 30:60:2:5. Moreover, we found that a step-terrace structure of Ca-Si-based oxides formed at the boundary of the M-type ferrite grain, and that the maximum width of the intergranular grain boundary was about half (1.15 nm) the edge length of the c-axis of the M phase. This suggested that ferrite grains were magnetically isolated by the presence of Ca-Si-based oxide phases at the intergranular grain boundary, whereby improving coercivity.