A heat treatment apparatus for a heat treatment of steel materials, which is at least one of a bright heat treatment, a thermal refining treatment, and a hardening/tempering treatment, comprising: a heat treatment furnace that heat-treats materials to be treated; a gas supply device that supplies atmosphere gas to the heat treatment furnace; a control system that controls the gas supply device; characterized in that the control system comprises a standard formation Gibbs energy computation unit configured to calculate standard formation Gibbs energy (ΔG 0) of the atmosphere gas in the heat treatment furnace by referring to sensor information from a CO 2 sensor and a temperature sensor; a display data generation unit configured to use an Ellingham diagram corresponding to the material in the heat treatment furnace and the calculated standard formation Gibbs energy to generate display data to be displayed on the display device, the display data generation unit generating the display data such that a status of the calculated standard formation Gibbs energy is displayed, together with a control range, on the Ellingham diagram on a display screen of the display device and is updated in real time on the display screen; and a control unit configured to control the gas supply device such that the calculated standard formation Gibbs energy enters the control range set on the Ellingham diagram, wherein the control range is in a region set below the approximate straight line as which standard formation Gibbs energy of the materials to be treated at respective temperatures is displayed, and is in a region set below the approximate straight line as which standard formation Gibbs energy in the reaction of 2C + O 2 = 2CO is displayed, wherein the control range includes: a first control range indicative of a normal operation range of the heat treatment furnace; a second control range outside the first control range; and a third control range outside the second control range, wherein the control system further comprises a status monitoring & abnormality processing unit configured to monitor the status of the calculated standard formation Gibbs energy on the Ellingham diagram, and wherein the status monitoring & abnormality processing unit is configured to output an alarm when the status deviates from the first control range, and to output control information so as to stop the operation of the heat treatment apparatus when the status shifts to the third control range. a heat treatment furnace that heat-treats materials to be treated; a gas supply device that supplies atmosphere gas to the heat treatment furnace; a control system that controls the gas supply device; a standard formation Gibbs energy computation unit configured to calculate standard formation Gibbs energy (ΔG 0) of the atmosphere gas in the heat treatment furnace by referring to sensor information from a CO 2 sensor and a temperature sensor; a display data generation unit configured to use an Ellingham diagram corresponding to the material in the heat treatment furnace and the calculated standard formation Gibbs energy to generate display data to be displayed on the display device, the display data generation unit generating the display data such that a status of the calculated standard formation Gibbs energy is displayed, together with a control range, on the Ellingham diagram on a display screen of the display device and is updated in real time on the display screen; and a control unit configured to control the gas supply device such that the calculated standard formation Gibbs energy enters the control range set on the Ellingham diagram, wherein the control range is in a region set below the approximate straight line as which standard formation Gibbs energy of the materials to be treated at respective temperatures is displayed, and is in a region set below the approximate straight line as which standard formation Gibbs energy in the reaction of 2C + O 2 = 2CO is displayed, wherein the control range includes: a first control range indicative of a normal operation range of the heat treatment furnace; a second control range outside the first control range; and a third control range outside the second control range, a first control range indicative of a normal operation range of the heat treatment furnace; a second control range outside the first control range; and a third control range outside the second control range, wherein the control system further comprises a status monitoring & abnormality processing unit configured to monitor the status of the calculated standard formation Gibbs energy on the Ellingham diagram, and wherein the status monitoring & abnormality processing unit is configured to output an alarm when the status deviates from the first control range, and to output control information so as to stop the operation of the heat treatment apparatus when the status shifts to the third control range.