Reversible, thermally controlled, spontaneous magnetization switching (reversible TCSMS) is demonstrated in the absence and presence of an external magnetic field in polycrystalline perovskite-type CaMn0.95Sb0.05O3. The spontaneous magnetization value is retained throughout a cyclic process even in bias magnetic fields. Neutron diffraction and theoretical studies indicate that two weak ferromagnetic phases, namely, spin-canted G- and A-type antiferromagnetic phases, coexist and that their ratio changes during magnetization switching. This spin structure transition is attributable to growth of eg orbital ordering clusters. The coexistence of two weak ferromagnetic phases plays a key role in exhibiting reversible TCSMS.