A surface hardening treatment device for performing a gas nitriding treatment or a gas nitrocarburizing treatment as a surface hardening treatment for a work made of metal arranged in a processing furnace by introducing a plurality of furnace introduction gases into the processing furnace, the plurality of furnace introduction gases including only an ammonia gas, only an ammonia decomposition gas, or only an ammonia gas and an ammonia decomposition gas, as a gas that produces hydrogen in the processing furnace, the surface hardening treatment device comprising an in-furnace atmospheric gas concentration detector configured to detect a hydrogen concentration or an ammonia concentration in the processing furnace, an in-furnace nitriding potential calculator configured to calculate a nitriding potential in the processing furnace based on the hydrogen concentration or the ammonia concentration detected by the in-furnace atmospheric gas concentration detector, and a gas-introduction-amount controller (21,22) configured to control an introduction amount of each of the plurality of furnace introduction gases by changing a flow rate ratio between the plurality of furnace introduction gases while keeping a total introduction amount of the plurality of furnace introduction gases constant, based on the nitriding potential in the processing furnace calculated by the in-furnace nitriding potential calculator and a target nitriding potential, such that the nitriding potential in the processing furnace is brought close to the target nitriding potential, wherein the target nitriding potential is set to be different values between time zones for the same work made of metal, but to be a constant value within each time zone, the gas-introduction-amount controller (21,22) is configured to perform a PID control method in which the gas introduction amounts of the plurality of furnace introduction gases are input values, the nitriding potential in the processing furnace calculated by the in-furnace nitriding potential calculator is an output value, and the target nitriding potential is a target value, and a proportional gain in the PID control method, an integral gain or an integration time in the PID control method, and a differential gain or a differentiation time in the PID control method can be set for each different value of the target nitriding potential, from among candidate values that have been obtained in advance by performing pilot processes. an in-furnace atmospheric gas concentration detector configured to detect a hydrogen concentration or an ammonia concentration in the processing furnace, an in-furnace nitriding potential calculator configured to calculate a nitriding potential in the processing furnace based on the hydrogen concentration or the ammonia concentration detected by the in-furnace atmospheric gas concentration detector, and a gas-introduction-amount controller (21,22) configured to control an introduction amount of each of the plurality of furnace introduction gases by changing a flow rate ratio between the plurality of furnace introduction gases while keeping a total introduction amount of the plurality of furnace introduction gases constant, based on the nitriding potential in the processing furnace calculated by the in-furnace nitriding potential calculator and a target nitriding potential, such that the nitriding potential in the processing furnace is brought close to the target nitriding potential, wherein the target nitriding potential is set to be different values between time zones for the same work made of metal, but to be a constant value within each time zone, the gas-introduction-amount controller (21,22) is configured to perform a PID control method in which the gas introduction amounts of the plurality of furnace introduction gases are input values, the nitriding potential in the processing furnace calculated by the in-furnace nitriding potential calculator is an output value, and the target nitriding potential is a target value, and a proportional gain in the PID control method, an integral gain or an integration time in the PID control method, and a differential gain or a differentiation time in the PID control method can be set for each different value of the target nitriding potential, from among candidate values that have been obtained in advance by performing pilot processes.