Activated magnetite ($Fe_3O_{4-{delta}}$) was applied to reducing $CO_2$ gas emissions to avoid greenhouse effects. Wet and dry methods were developed as a $CO_2$ removal process. One of the typical dry methods is $CO_2$ decomposition using activated magnetite ($Fe_3O_{4-{delta}}$). Generally, $Fe_3O_{4-{delta}}$ is manufactured by reduction of $Fe_3O_4$ by $H_2$ gas. This process has an explosion risk. Therefore, a non-explosive process to make $Fe_3O_{4-{delta}}$ was studied using $FeC_2O_4{cdot}2H_2O$ and $N_2$. $FeSO_4{cdot}7H_2O$ and $(NH_4)_2C_2O_4{cdot}H_2O$ were used as starting materials. So, ${alpha}-FeC_2O_4{cdot}2H_2O$ was synthesized by precipitation method. During the calcination process, $FeC_2O_4{cdot}2H_2O$ was decomposed to $Fe_3O_4$, CO, and $CO_2$. The specific surface area of the activated magnetite varied with the calcination temperature from 15.43 $m^2/g$ to 9.32 $m^2/g$. The densities of $FeC_2O_4{cdot}2H_2O$ and $Fe_3O_4$ were 2.28 g/$cm^3$ and 5.2 g/$cm^3$, respectively. Also, the $Fe_3O_4$ was reduced to $Fe_3O_{4-{delta}}$ by CO. From the TGA results in air of the specimen that was calcined at $450^{circ}C$ for three hours in $N_2$ atmosphere, the ${delta}$-value of $Fe_3O_{4-{delta}}$ was estimated. The ${delta}$-value of $Fe_3O_{4-{delta}}$ was 0.3170 when the sample was heat treated at $400^{circ}C$ for 3 hours and 0.6583 when the sample was heat treated at $450^{circ}C$ for 3 hours. $Fe_3O_{4-{delta}}$ was oxidized to $Fe_3O_4$ when $Fe_3O_{4-{delta}}$ was reacted with $CO_2$ because $CO_2$ is decomposed to C and $O_2$.