Porous manganese oxide porous nanostructures were prepared by amino-acid-mediated solvothermal self assembly reaction and subsequent heat treatment at $600^{circ}C$. When Mn-precursors were heat-treated at $400-550^{circ}C$, the sensors did not show significant gas responses. In contrast, the manganese oxide heat-treated at $600^{circ}C$ showed the significant gas responses, that is, the resistance decrease to 100 ppm $C_3H_8$ ($R_a/R_g$ = 2.17, $R_a$ : resistance in air, $R_g$ : resistance in gas) and the resistance increase to 100 ppm $C_2H_5OH$ ($R_g/R_a$ = 1.92). The opposite change of resistance upon exposure to $C_3H_8$ and $C_2H_5OH$ was discussed in relation to the mixed phases of manganese oxides with different valences.