To provide trimanganese tetraoxide from which a lithium manganese oxide with less fusion of particles by firing, i.e. necking phenomenon, is obtained. Trimanganese tetraoxide characterized in that the pore volume of pores having 5 pore diameters of from 0.3 to 2 μm is at least 0.1 mL/g. The pore volume of pores having pore diameters of from 0.5 to 1 μm is preferably at least 0.03 mL/g.