To provide a die for aluminum extrusion having a nitriding layer excellent in surface hardness and wear resistance.

At the time of subjecting a hot tool steel to quenching and tempering treatment, thereafter, removing an oxide layer formed on the surface of the hot tool steel by mechanical treatment and/or chemical treatment and subjecting the same to nitriding treatment of executing holding at 480 to 550°C for 5 to 10hr in a gas atmosphere for nitriding having an [N] source, the concn. of oxygen in the atmosphere in the process of the chemical treatment or nitriding treatment is regulated to 20 to 100ppm so as to regulate the X-ray intensity ratio of Fe3O4/Fe3N in the surface of the hot tool steel to ≤5%. Since a nitriding layer in which iron oxide is suppressed is formed, its elimination, peeling or the like can be suppressed to prolong the service life of the die.