In this work, pressure dependence of hydrogen for temperature of heat generated by the exothermic hydrogenation of magnesium (Mg) was investigated. When hydrogen was introduced into the reactor at 0.7 MPa, the temperature of the reactor was increased to approximately 350°C, which is the theoretical equilibrium temperature at this pressure, within only 10 seconds. Thus, the temperature increased by heat generation during the hydrogenation of Mg is highly responsive to pressure, suggesting that the temperature can be controlled by hydrogen pressure.