A general synthetic method to make $Fe_3O_{4-{delta}}$ (activated magnetite) is the reduction of $Fe_3O_4$ by $H_2$ atmosphere. However, this process has an explosion risk. Therefore, we studied the process of synthesis of $Fe_3O_{4-{delta}}$ depending on heat-treatment conditions using $FeC_2O_4{cdot}2H_2O$ in Ar atmosphere. The thermal decomposition characteristics of $FeC_2O_4{cdot}2H_2O$ and the ${delta}$-value of $Fe_3O_{4-{delta}}$ were analyzed with TG/DTA in Ar atmosphere. ${eta}-FeC_2O_4{cdot}2H_2O$ was synthesized by precipitation method using $FeSO_4{cdot}7H_2O$ and $(NH_4)_2C_2O_4{cdot}H_2O$. The concentration of the solution was 0.1 M and the equivalent ratio was 1.0. ${eta}-FeC_2O_4{cdot}2H_2O$ was decomposed to $H_2O$ and $FeC_2O$4 from $150^{circ}C$ to $200^{circ}C$. $FeC_2O4$ was decomposed to CO, $CO_2$, and $Fe_3O_4$ from $200^{circ}C$ to $250^{circ}C$. Single phase $Fe_3O_4$ was formed by the decomposition of ${eta}-FeC_2O_4{cdot}2H_2O$ in Ar atmosphere. However, $Fe_3C$, Fe and $Fe_4N$ were formed as minor phases when ${eta}-FeC_2O_4{cdot}2H_2O$ was decomposed in $N_2$ atmosphere. Then, $Fe_3O_4$ was reduced to $Fe_3O_{4-{delta}}$ by decomposion of CO. The reduction of $Fe_3O_4$ to $Fe_3O_{4-{delta}}$ progressed from $320^{circ}C$ to $400^{circ}C$; the reaction was exothermic. The degree of exothermal reaction was varied with heat treatment temperature, heating rate, Ar flow rate, and holding time. The ${delta}$-value of $Fe_3O_{4-{delta}}$ was greatly influenced by the heat treatment temperature and the heating rate. However, Ar flow rate and holding time had a minor effect on ${delta}$-value.