Provided are a surface modifier for an alloy steel component, a surface modification method for an alloy steel component, and a method for producing an alloy steel component which can provide a deep and homogeneous hard layer in a steel component having a wide shape or a large number of such steel components. A surface modification apparatus 100 includes a process furnace 101, and the process furnace 101 performs a surface modification process on an alloy steel component 90 including an alloy steel material to which at least one kind of nitriding member such as chromium, molybdenum, or aluminum has been added. The surface modification apparatus 100 forms a bonding layer on a surface of the alloy steel component 90 by exposing the alloy steel component 90 for at least 180 minutes to an atmosphere in the process furnace 101 whose ammonia gas concentration is maintained at 80% or higher and temperature at 620 ° C.
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